I put this comment elsewhere in the thread, but maybe if I put it in this subthread you'll get to see it.
I tested on my laptop (a ThinkPad X250) and alacritty is slower than xterm. xterm can display find / at 80x24 in 11 seconds, but alacritty takes 17 seconds or more, depending on how large the window is (smaller seems to be slower) and whether it's on-screen or not (off-screen seems to be slower).
There's a small subset of systems experiencing this. Do you happen to have a Radeon video card? In the profile I looked at, glClear was calling down into (through libxcb) __poll_nocancel which was eating 99% of the CPU time. I'm not sure if there's an issue open for this yet, but it's something we're looking into. One of my testers during development ran into this so we're aware of the problem.
I tested on my laptop (a ThinkPad X250) and alacritty is slower than xterm. xterm can display find / at 80x24 in 11 seconds, but alacritty takes 17 seconds or more, depending on how large the window is (smaller seems to be slower) and whether it's on-screen or not (off-screen seems to be slower).