> most apps can relayout and redraw their window in a tiny fraction of the time that animation takes
Re-rendering to the same compositor target buffer (when you e.g. resize a window), and allocating new textures in (potentially) new formats under control of a new compositor target buffer, are different orders-of-magnitudes of work. Think about how much delay there is from the point you plug in an external display, to the point your windows finish "popping over" onto it and it becomes usable. Allocating a new virtual desktop and moving a window to become fullscreen on it, is doing 90% of that same work.
Why does each virtual desktop have its own compositor and own target buffer? Because each virtual desktop has its own active display mode it's operating under: its own display resolution, its own bitplane format [think RGB vs BGR displays], different memory allocation constraints depending on whether it's currently being mirrored / screen-shared or not, maybe allocated in the VRAM of a different GPU if it's being drawn on a monitor plugged into a discrete GPU's HDMI/DP socket, etc.
This "distinct target buffers with their own compositors" approach is also why you can't span windows across displays on macOS 10.9+. (Before that, macOS used a different approach, that allowed for single virtual desktops that span multiple monitors, but where this would use one big target buffer, formatted to the lowest-common denominator format acceptable to all display targets. So if you plugged in a shitty external display like a 720p projector, and "extended" your desktop to it, then the whole compositor target buffer would get reformatted, and so your internal display's resolution/color depth/refresh rate would decrease.)
> Anyway, I'd trade a bit of pop-in for a faster computer if that was necessary.
My point is that this wouldn't be the trade-off. Instead, you'd switch over to the new fullscreen view, half-rendered — and then it would very likely still take just as long to finish re-rendering and accept input as if there was no animation.
You can actually try this "experience" out for yourself: try having an external display connected; opening Mission Control; and then dragging the focused virtual desktop from your internal display, over to the external display (or vice-versa.) It takes about 3 seconds before all texture assets finish re-allocating in the new compositor target buffer's texture format and re-rendering; and because there's no animation to paper over this case, it just feels unresponsive, like the whole shell has stalled for 3 seconds.
