I have Raspberry Pi 4B 4 GB model, and I installed Ubuntu 64bit. When I watch YouTube with 1080p, it lags a lot. I tried many things, I tried splitting memory, using Chromium, I set Chromium to use GPU but it's still slow and lags a lot. CPU is 100% for all cores, and temperature is really hot.
I didn't overclocked RPi 4.
Note: I already tried memory split (i.e. changing the amount of RAM available to the GPU), but it was still slow.