Fps drops from 60 to 20 after going fullscreen while still showing 60 ingame

Hello there. I originally posted this on gaming, but since I am convinced that it has nothing to do with game itself, there it goes.

I stumbled upon a problem that when I increase window size or even go fullscreen in minecraft framerate gradually drops, until 15-20 in fullscreen, although ingame debug overlay still shows same amount of fps. (Tried running 120 fps: feels exactly the same (20))
I tried to run different versions. Same result. The only thing I suspect now is Xorg/mesa messing with resolution/frames or so, idk.

Dependancies (if necessary):
jre-openjdk 17.0.1.u12-1
java-runtime-common 3-3

inxi output:

System:
  Kernel: 5.13.19-2-MANJARO x86_64 bits: 64 compiler: gcc v: 11.1.0
Desktop: KDE Plasma 5.23.3 tk: Qt 5.15.2 wm: kwin_x11 vt: 1 dm: SDDM
  Distro: Manjaro Linux base: Arch Linux
CPU:
  Info: Dual Core model: Intel Core i5-2390T bits: 64 type: MT MCP
  arch: Sandy Bridge family: 6 model-id: 2A (42) stepping: 7 microcode: 2F
  cache: L1: 128 KiB L2: 512 KiB L3: 3 MiB bogomips: 21559
  Speed: 3137 MHz min/max: 1600/3500 MHz Core speeds (MHz): 1: 3137 2: 3078
  3: 3153 4: 2234
Graphics:
  Device-1: AMD Caicos [Radeon HD 6450/7450/8450 / R5 230 OEM]
  vendor: Pegatron driver: radeon v: kernel bus-ID: 01:00.0 chip-ID: 1002:6779
  class-ID: 0300
Display: x11 server: X.Org 1.21.1.1 compositor: kwin_x11 driver:
  loaded: ati,radeon unloaded: modesetting alternate: fbdev,vesa
  display-ID: :0 screens: 1
  Screen-1: 0 s-res: 1920x1080 s-dpi: 96 s-size: 508x285mm (20.0x11.2")
  s-diag: 582mm (22.9")
  Monitor-1: LVDS res: 1920x1080 hz: 60 dpi: 96 size: 509x286mm (20.0x11.3")
  diag: 584mm (23")
  OpenGL: renderer: AMD CAICOS (DRM 2.50.0 / 5.13.19-2-MANJARO LLVM 13.0.0)
  v: 3.3 Mesa 21.2.5 compat-v: 3.1 direct render: Yes

glxinfo -B:

name of display: :0
display: :0  screen: 0
direct rendering: Yes
Extended renderer info (GLX_MESA_query_renderer):
    Vendor: X.Org (0x1002)
    Device: AMD CAICOS (DRM 2.50.0 / 5.13.19-2-MANJARO, LLVM 13.0.0) (0x6779)
    Version: 21.2.5
    Accelerated: yes
    Video memory: 1024MB
    Unified memory: no
    Preferred profile: core (0x1)
    Max core profile version: 3.3
    Max compat profile version: 3.1
    Max GLES1 profile version: 1.1
    Max GLES[23] profile version: 3.1
Memory info (GL_ATI_meminfo):
    VBO free memory - total: 1023 MB, largest block: 1023 MB
    VBO free aux. memory - total: 1021 MB, largest block: 1021 MB
    Texture free memory - total: 1023 MB, largest block: 1023 MB
    Texture free aux. memory - total: 1021 MB, largest block: 1021 MB
    Renderbuffer free memory - total: 1023 MB, largest block: 1023 MB
    Renderbuffer free aux. memory - total: 1021 MB, largest block: 1021 MB
Memory info (GL_NVX_gpu_memory_info):
    Dedicated video memory: 1024 MB
    Total available memory: 2045 MB
    Currently available dedicated video memory: 1023 MB
OpenGL vendor string: X.Org
OpenGL renderer string: AMD CAICOS (DRM 2.50.0 / 5.13.19-2-MANJARO, LLVM 13.0.0)
OpenGL core profile version string: 3.3 (Core Profile) Mesa 21.2.5
OpenGL core profile shading language version string: 3.30
OpenGL core profile context flags: (none)
OpenGL core profile profile mask: core profile

OpenGL version string: 3.1 Mesa 21.2.5
OpenGL shading language version string: 1.40
OpenGL context flags: (none)

OpenGL ES profile version string: OpenGL ES 3.1 Mesa 21.2.5
OpenGL ES profile shading language version string: OpenGL ES GLSL ES 3.10

I also tried to play in xorg.conf.d with different options (TearFree and everything related to vsync). Nothing seemed changed (I hope I tried it correct way) .

How do I fix it?