*UPDATE
Switching to Integrated GPU did not help, issues and bugs still occurring for KDE 5.26.2 on my side.
Kwin unresponsiveness and Plasma freezing occur when my stacked notifications arrive to the desktop after lit up the laptop. It seems Plasma and Kwin cannot handle stacked tasks when all come in a short period.
This one’s fresh as of today. 
Hoo boy. I suppose the Nouveau driver is equally vulnerable then. (Which is what I am using.)
*2nd UPDATE
I’ve gone back to Kernel 5.15.76-1 yesterday and still on Integrated GPU also disabled desktop notifications. Today when I lit up my laptop from sleep mode after hours no any glitch, unresponsiveness or freezing.
In normally, later than update of KDE 5.26.2 I could not use my laptop after stacked notifications appeared on my screen, KDE was going to frozen mode what even CTRL + ALT + F2/F3… not responding.
Issues seems related to Notifications-Kernel-GPU combinations.
Weird but disabling notifications yeah that helped me.
System:
Kernel: 5.15.76-1-MANJARO arch: x86_64 bits: 64 compiler: gcc v: 12.2.0
parameters: BOOT_IMAGE=/boot/vmlinuz-5.15-x86_64
root=UUID=278a86b1-7436-448c-929d-fa62c972f0b1 rw ipv6.disable=1 quiet
apparmor=1 security=apparmor udev.log_priority=3 acpi_backlight=vendor
Desktop: KDE Plasma v: 5.26.2 tk: Qt v: 5.15.6 wm: kwin_x11 vt: 1 dm: SDDM
Distro: Manjaro Linux base: Arch Linux
Machine:
Type: Laptop System: Acer product: Nitro AN515-44 v: V1.04
serial: <superuser required>
Mobo: RO model: Stonic_RNS v: V1.04 serial: <superuser required>
UEFI: Insyde v: 1.04 date: 02/04/2021
Battery:
ID-1: BAT1 charge: 42.0 Wh (79.4%) condition: 52.9/58.8 Wh (90.0%)
volts: 16.4 min: 15.4 model: SMP AP18E7M type: Li-ion serial: <filter>
status: N/A
CPU:
Info: model: AMD Ryzen 7 4800H with Radeon Graphics bits: 64 type: MT MCP
arch: Zen 2 gen: 3 level: v3 note: check built: 2020-22
process: TSMC n7 (7nm) family: 0x17 (23) model-id: 0x60 (96) stepping: 1
microcode: 0x8600103
Topology: cpus: 1x cores: 8 tpc: 2 threads: 16 smt: enabled cache:
L1: 512 KiB desc: d-8x32 KiB; i-8x32 KiB L2: 4 MiB desc: 8x512 KiB L3: 8 MiB
desc: 2x4 MiB
Speed (MHz): avg: 1894 high: 3299 min/max: 1400/2900 boost: enabled
scaling: driver: acpi-cpufreq governor: performance cores: 1: 2915 2: 1397
3: 1808 4: 1515 5: 2484 6: 1583 7: 1397 8: 1885 9: 3130 10: 1497 11: 3299
12: 1398 13: 1453 14: 1755 15: 1397 16: 1398 bogomips: 92662
Flags: avx avx2 ht lm nx pae sse sse2 sse3 sse4_1 sse4_2 sse4a ssse3 svm
Vulnerabilities:
Type: itlb_multihit status: Not affected
Type: l1tf status: Not affected
Type: mds status: Not affected
Type: meltdown status: Not affected
Type: mmio_stale_data status: Not affected
Type: retbleed mitigation: untrained return thunk; SMT enabled with STIBP
protection
Type: spec_store_bypass mitigation: Speculative Store Bypass disabled via
prctl and seccomp
Type: spectre_v1 mitigation: usercopy/swapgs barriers and __user pointer
sanitization
Type: spectre_v2 mitigation: Retpolines, IBPB: conditional, STIBP:
always-on, RSB filling, PBRSB-eIBRS: Not affected
Type: srbds status: Not affected
Type: tsx_async_abort status: Not affected
Graphics:
Device-1: AMD Renoir vendor: Acer Incorporated ALI driver: amdgpu v: kernel
arch: GCN-5.1 code: Vega-2 process: TSMC n7 (7nm) built: 2018-21 pcie:
gen: 4 speed: 16 GT/s lanes: 16 ports: active: eDP-1 empty: none
bus-ID: 05:00.0 chip-ID: 1002:1636 class-ID: 0300 temp: 40.0 C
Device-2: Quanta HD User Facing type: USB driver: uvcvideo bus-ID: 3-3:3
chip-ID: 0408:a061 class-ID: 0e02
Display: x11 server: X.Org v: 21.1.4 compositor: kwin_x11 driver: X:
loaded: amdgpu unloaded: modesetting alternate: fbdev,vesa dri: radeonsi
gpu: amdgpu display-ID: :0 screens: 1
Screen-1: 0 s-res: 1920x1080 s-dpi: 96 s-size: 508x285mm (20.00x11.22")
s-diag: 582mm (22.93")
Monitor-1: eDP-1 mapped: eDP model: Najing CEC Panda 0x004d built: 2019
res: 1920x1080 dpi: 142 gamma: 1.2 size: 344x194mm (13.54x7.64")
diag: 395mm (15.5") ratio: 16:9 modes: max: 1920x1080 min: 640x480
API: OpenGL v: 4.6 Mesa 22.2.1 renderer: RENOIR (renoir LLVM 14.0.6 DRM
3.42 5.15.76-1-MANJARO) direct render: Yes
Audio:
Device-1: AMD ACP/ACP3X/ACP6x Audio Coprocessor
vendor: Acer Incorporated ALI driver: N/A alternate: snd_pci_acp3x,
snd_rn_pci_acp3x, snd_pci_acp5x pcie: gen: 4 speed: 16 GT/s lanes: 16
bus-ID: 05:00.5 chip-ID: 1022:15e2 class-ID: 0480
Device-2: AMD Family 17h/19h HD Audio vendor: Acer Incorporated ALI
driver: snd_hda_intel v: kernel pcie: gen: 4 speed: 16 GT/s lanes: 16
bus-ID: 05:00.6 chip-ID: 1022:15e3 class-ID: 0403
Sound API: ALSA v: k5.15.76-1-MANJARO running: yes
Sound Server-1: JACK v: 1.9.21 running: no
Sound Server-2: PulseAudio v: 16.1 running: yes
Sound Server-3: PipeWire v: 0.3.59 running: yes
Network:
Device-1: Realtek vendor: Acer Incorporated ALI driver: r8169 v: kernel
pcie: gen: 1 speed: 2.5 GT/s lanes: 1 port: 2000 bus-ID: 03:00.0
chip-ID: 10ec:2600 class-ID: 0200
IF: enp3s0 state: down mac: <filter>
Device-2: Intel Wi-Fi 6 AX200 driver: iwlwifi v: kernel pcie: gen: 2
speed: 5 GT/s lanes: 1 bus-ID: 04:00.0 chip-ID: 8086:2723 class-ID: 0280
IF: wlp4s0 state: up mac: <filter>
Bluetooth:
Device-1: Intel AX200 Bluetooth type: USB driver: btusb v: 0.8 bus-ID: 1-4:2
chip-ID: 8087:0029 class-ID: e001
Report: rfkill ID: hci0 rfk-id: 7 state: down bt-service: enabled,running
rfk-block: hardware: no software: yes address: see --recommends
Drives:
Local Storage: total: 476.94 GiB used: 343.87 GiB (72.1%)
SMART Message: Unable to run smartctl. Root privileges required.
ID-1: /dev/nvme0n1 maj-min: 259:0 vendor: Western Digital model: PC SN530
SDBPNPZ-512G-1014 size: 476.94 GiB block-size: physical: 512 B
logical: 512 B speed: 31.6 Gb/s lanes: 4 type: SSD serial: <filter>
rev: 21103900 temp: 30.9 C scheme: GPT
Partition:
ID-1: / raw-size: 153.02 GiB size: 149.56 GiB (97.74%)
used: 117.9 GiB (78.8%) fs: ext4 dev: /dev/nvme0n1p4 maj-min: 259:4
ID-2: /boot/efi raw-size: 1024 MiB size: 1020 MiB (99.61%)
used: 43.4 MiB (4.3%) fs: vfat dev: /dev/nvme0n1p1 maj-min: 259:1
Swap:
Kernel: swappiness: 10 (default 60) cache-pressure: 50 (default 100)
ID-1: swap-1 type: partition size: 2 GiB used: 55.1 MiB (2.7%)
priority: -2 dev: /dev/nvme0n1p5 maj-min: 259:5
Sensors:
System Temperatures: cpu: 58.5 C mobo: N/A gpu: amdgpu temp: 40.0 C
Fan Speeds (RPM): N/A
Info:
Processes: 421 Uptime: 23h 40m wakeups: 6 Memory: 15 GiB
used: 7.84 GiB (52.3%) Init: systemd v: 251 default: graphical
tool: systemctl Compilers: gcc: 12.2.0 clang: 14.0.6 Packages: 1651
pm: pacman pkgs: 1607 libs: 412 tools: pamac pm: flatpak pkgs: 38 pm: snap
pkgs: 6 Shell: Zsh v: 5.9 default: Bash v: 5.1.16 running-in: konsole
inxi: 3.3.23
OS: Manjaro Linux x86_64
██████████████████ ████████ Host: Nitro AN515-44 V1.04
████████ ████████ Kernel: 5.15.76-1-MANJARO
████████ ████████ ████████ Uptime: 23 hours, 28 mins
████████ ████████ ████████ Packages: 1607 (pacman), 38 (flatpak), 6 (snap)
████████ ████████ ████████ Shell: bash 5.1.16
████████ ████████ ████████ Resolution: 1920x1080
████████ ████████ ████████ DE: Plasma 5.26.2
████████ ████████ ████████ WM: KWin
████████ ████████ ████████ WM Theme: Nordic
████████ ████████ ████████ Theme: Breath2 2021 Dark [Plasma], WhiteSur-Dark [GTK2/3]
████████ ████████ ████████ Icons: kora [Plasma], kora [GTK2/3]
Terminal: konsole
CPU: AMD Ryzen 7 4800H with Radeon Graphics (16) @ 2.900GHz
GPU: AMD ATI 05:00.0 Renoir
Memory: 7521MiB / 15362MiB
I temporarily “fixed” the issue, by completely disabling all plasma notifications.
Simply turn on don’t disturb until you manually enable it again.
I have been following this thread and I find it rather strange
What is your differences?
What is the difference betwenn my system and yours?
Let me layout my system
The same problems are being reported on new installations, and on existing installations without any custom theming at all.
While it is true that some themes would be problematic because they were created for Plasma versions older than 5.12, newer themes should be fully compatible, and so should widgets be that were created by the KDE developers themselves.
If Plasma were intended to be used without any customizations at all, then the KDE developers should themselves not offer any means to customize it. 
I agree on the contradictons - but sometimes there’s very little to go on 
Well, it depends which car you drive. If we compare software with cars you see similar things.
In the 60s cars were simple. Drove you from A to B and there were few models to choose from. You had the gasoline and Diesel engine. Both invented in Germany. In the 80s the tuner scene was born so people pimp up their vehicles. Later on new engines like electric and hydrogen was born. All those are more driven automatic, which is also a huge factor in the states. Only geeks drive with gears these days.
From the technical point of view, the old engines were more complex but also easier to repair. The new stuff is more combined with electronic and computers. All due to innovation and having new features.
If we put that now to Software and especially to KDE we saw many iterations of itself and how it envolved. The Kool Desktop from Germany made it mainstream. And with other desktop environments out there having a standard is more complex as you’re not alone to drive it forward. Also a lot of influence is happening from outside - community and other projects. So KDE wants still to be Kool. However they changed their slogan to “Simple by default, powerful when needed” overtime.
KDE with its approach in Frameworks, Plasma and Apps (Gear) is a nice construct. Especially Frameworks is used in other projects like LxQt and JingOS. The Apps you find also in Microsoft Store and other platforms. Plasma is great to use. Also lately Companies using KDE software more often. Partly also cos Qt is some sort of industry standard and backed by a company. So if it is user and community driven you expect more features out of it. Corporate driven it will be more focused on the needs of a company and stability. Plasma LTS finds a great middle way for that. Kubuntu and Suse Leap a great examples of solid distros using LTS branches.
Lately it is not all rosy. Also Gnome has its ups and downs. As soon as you add extensions the shell might break if 3rd party developers don’t catch up with changes made by the upstream project. You also see those split-off projects like Mate, Cinnamon, Budgie and many more. FOSS is great, as if you don’t agree you fork or create some new.
Maui-Shell is using KDE Frameworks but add some own UI for the Nitrux Distribution. Plasma Mobile brings Plasma on Phone and Tablets and Plasma Bigscreen to the TV. However new ways may need new designs which creates also fragmentation within the project. So the current focus is still more on desktop side but also on mobile due to the SteamDeck. Maui-Shell tries to be a glue and adaptive to be used on Mobile and on Desktop with the same UI design. Phosh and Gnome on Mobile try to achieve the same with libhandy and libadwaita. With each new tech new ways of how you use your PC will happen. You either participate with those changes and report issues early or accept that changes might take more time to establish the used stability it once has when the focus was only on one thing - the desktop.
XFCE is not fancy, doesn’t has many features but it is stable and simple as its community and developers only focus on minimal improvements and changes to keep it solid.
So ya - software can always be fun to use. If you keep it close to the design of its developers and use similar hardware as they do you’re fine. If you more into adventures you have to step up and give back - at least with documentation of the issues you may have.
Also there is the tend to talk more about what is not working or got broken rather what got improved or new invented. Humans simply have a small problem to adjust to changes. We like all our established habits …
@philm - You are spot-on - exactly my point.
While it is possible to apply a different theme, add extensions to the kernel add specialised graphics and engines targeted gaming
Plasma is a framework upon which componenents are added using layout definitions and qt-script (based on ecmascript) - so when a component uses framework calls which has been altered or deprecated the system no longer function as expected.
Such framework changes reaches the user when the system is synced with newer packages and as such the user blames the sync and thus the distribution.
The reality is a lot more nuanced as it could very well be the theme or the widget which has to be rebuilt to new framework.
people using KDE for theming and customization ability, otherwise what’s the matter the changing DE?
And that is fine by me - my point is - knowledge - know your system - be prepared to troubleshoot your customisations - Plasma developers does not test each and every theme for backwards compatibility - that is a task for the maintainer of the theme or if unmaintained - the user owning the system.
That somehow is a typical american thing to say - for classic fuel type vehicles at least. 
Only people getting many notifications seem to be affected - that’s why you might not see it.
Apparently 5.26.x keeps on biting people. The following user is using the default theme, by the way, so there’s no point in shoving the blame onto customization anymore. 
Interesting… Probably just the (un)luckiness of the draw then…? 
I am having problems with 5.26.2 as well, when booting up with a second monitor connected, my system fonts are wrong, my taskbar is just completely green, many items (As the ones on the desktop or in the application launcher) dont have icons anymore, …
I hope the next updates address this, its pretty much unusable atm.
5.26.3 has just entered the Testing branch right now, so it won’t be long anymore before it hits Stable.
No guarantee that it’ll fix things, of course.
Just as long as it isn’t…a direct hit…! 
I had zero issues with plasma 5.25, however the update to 5.26 caused some problems, see All Icons are missing. A downgrade to 5.24.7-LTS (thanks for creating this!) solved the issue.
I was running 5.25.x, I noticed no issues.
I’m now on 5.26.2, the only issue I’ve had was 5.26.2 seemed to ignore the Window rules I’d set on 5.25.x… but only for some windows.
I simply deleted the old rules and recreated new rules.
Other than that, everything seems boringly normal.
Operating System: Manjaro Linux
KDE Plasma Version: 5.26.2
KDE Frameworks Version: 5.99.0
Qt Version: 5.15.6
Kernel Version: 5.15.76-1-MANJARO (64-bit)
Graphics Platform: X11
Processors: 4 × Intel® Pentium® CPU N3540 @ 2.16GHz
Memory: 7.6 GiB of RAM
Graphics Processor: Mesa Intel® HD Graphics
Manufacturer: Notebook
Product Name: W54_W94_W955TU,-T,-C
System Version: Not Applicable