> Even something as simple as alt-tabbing lagged for seconds on an overpowered machine.

This may not be KDE's fault; I tracked these kinds of issues down to some bad tunable defaults.

I came up with this:

    ----
    cat /etc/sysctl.d/50-usb-responsiveness.conf
    #
    # Attempt to keep large USB transfers from locking the system (kswapd0)
    #
    vm.swappiness = 1
    vm.dirty_background_ratio = 5
    vm.dirty_ratio = 5
    vm.extfrag_threshold = 1000
    vm.compaction_proactiveness = 0
    vm.vfs_cache_pressure = 200
    # FIXME? 64K too big?
    vm.page-cluster = 16
    ----

I have fast everything, NVMe SSD onboard and others in Thunderbolt 4 enclosures and 32GB of RAM on my 12th-Gen i7 with 20 (6+14) cores; there should have been no reason for any stuttering and/or Alt-Tab slowness while doing large file copies and finally got fed up, did some research and experimentation and use the above and it's not happened since.

YMMV, but it's worth a try.

(Oh, and on-topic, I've had to try Wayland (vs. X11) on my KDE desktop 'cause it seems to handle switching monitors when I go from home to work better; jury's still out if I'm keeping it)