I have RPi4B and there’s no such an issue with fps in glxgears (over 1200 fps!):
It is slightly overclocked (if you would call 1,5→1,6 GHz an overclock), but with no overvoltage set.
Is this an issue then with 4K resolution only? Have you tried setting your monitor to lower resolutions and checked if there’s a fps gain? As you can see, I’m on 1440x900 resolution – quite small comparing to 4K. If it affects also the lower resolutions modes on your Pi, you could try my tips below.
What is your cmdline.txt? Did you set usbhid.mousepoll to 0 (if yes, better try higher values, I personally recommend 8). Also, you could try disabling the compositor (that’s all thing I do on my RPi after I installed on it Manjaro – except setting the usbhid.mousepoll since my mouse works fine with the default values). I know from practise that combination of vblank_mode=auto in XFCE on the Manjaro ARM + high mouse polling makes Pi quite slow.
Sorry if I’m saying the same I were on the old forum. Even if you’re using Plasma Desktop, setting mouse-polling might give you some fps up as it did for me on XFCE (and even on Raspberry Pi OS as I checked later).