They're both leaving practical performance on the table by not implementing VT420 (or better) control sequences, though. So whatever gains you get from them are lost by applications needing to do more work. For example, tmux uses DECLRMM to restrict scrolling to the active part of horizontally split panes, and as neither Alacritty nor Kitty implements DECLRMM tmux will have to do more work to get the same effect.

The author of Zutty has a writeup about it: https://tomscii.sig7.se/2020/12/A-totally-biased-comparison-...