alt Hacker NewsMy newer desktop (2020 era with a 3070) has 4x 4k monitors attached running XFCE and I have never noticed the lag you speak of. I don't run external monitors on it but my thinkpad x200 with a core 2 duo also does great with xfce.
I have no doubt the issues you speak of exist in theory but they do not seem to matter in practice.
You shouldn't notice lag. On modern Xorg the only round-trip is context switches between server and compositor, because the only thing what is shared is texture dma-bufs (there is inefficiency in mesa code for GLX_EXT_texture_from_pixmap extension, but it is other story). And if dma-bufs is working (Xorg needs to test and pull one MR) you have buffer direct scanouts as in wayland.