Alacritty's performance in day-to-day use is actually very underwhelming, I tried to use it but there was just so many little lags left and right I promptly switched back to urxvt
urxvt was my workhorse for years; And I tried Alacritty + a tmux configure that emulated urxvt's tabs. It wasn't quite all the way there, wezterm is another one that seems to work well and has slightly improved rendering over urxvt, and I'm pretty happy with it right now, though it has a lot more bells & whistles.
Alacritty is a different approach: they're building a traditional terminal. Warp is hoping to push the boundaries and build something different.
Warp is a fork of Alacritty, their team was very gracious in collaborating and reviewing Warp's initial designs.