Performance differences between xfce and KDE will be small. If you want a lean system you could also consider the minimal edition of KDE, won’t help in performance but in system size. For the performance the kind of your disk might make a difference especially for booting a SSD is a huge improvement compared to a HDD. Customizing in KDE is superior, Gnome is rather on the opposite.
I’ve tried both DE’s on Manjaro and to backup the initial response to your question on this thread, I agree there’s not a lot in it performance wise: XFCE is fractionally “snappier” in certain cases but not really faster.
I’m using KDE Plasma myself with an Intel Kaby Lake i3 7100 and integrated HD630 plus 8GB of DDR4 RAM clocked at 2133Mhz only…and it’s quick! And I’m using a 1TB Caviar Blue HDD and not an SSD. Boot up times will differ, however, depending on the desktop and hard-drive used.
I have an Intel i5-8400 “Coffee Lake” (6-core, no Hyperthreading) with Intel UHD 630. It has 16 GiB of DDR4 RAM, although it had only 8 GiB when I bought it. I also do have the system installed on an (SATA3-connected) SSD.
My boot time, from pressing Enter at the GRUB screen to the SDDM login screen, is around 10 seconds. It used to be 11, but I’ve customized my /etc/mkinitcpio.conf ─ which is used for building the initramfs ─ because I have my /usr on a separate partition, and the new configuration shaved one second off of the boot time.
As @Ben said, with the integrated Intel graphics, you can make ample use of all the eye-candy that KDE Plasma offers, from translucency and blur over drop-shadows to wobbly windows, the spinning desktop cube ─ in my case, it’s more of a carousel, because I have 12 virtual desktops ─ and lots more. And 8 GiB of RAM is plenty.