Hmm, I'd say that on a 2018-era machine, you won't measure this in raw CPU throughput. In all probablity, your cores are fast enough to mask the context switching. The performance deficit here is strictly in the domain of motion-to-photon latency or frame pacing. I guess my point is that in xfce's split architecture, the compositor acts as just another X11 client.
This enforces a path where window contents often round-trip through the X server before composition. Quantitatively, this typically adds at least one frame of input lag compared to the zero-copy direct scanout path available to monolithic wayland compositors. You likely won't notice this while editing text. However, the architecture doesn't perform well when you attach an external monitor. Since X11 shares a single virtual coordinate space, it cannot synchronize VBLANK across two outputs with different refresh rates or clock domains.
ps: and please don't call your 2018 machine vintage, it makes my secondary thinkpads feel prehistoric :D
My 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.
As someone who runs modern XFCE on a core 2 duo I still have without noticable perf issues, the problems the parent talks about are theoretical and not observable.
I am running XFCE on a 2019 vintage desktop. CachyOS and 16GB RAM. It is snappy and very performant for my needs and I work on it daily for software development
