go to https://codepen.io/aragakey/pen/LYmXqEp
Click to redraw/randomize the demo

ER: fast redraw
AR: Somewhat laggy redraw

Profile: https://share.firefox.dev/3ssiJIk

Chrome is also slow in this case so not sure if much time should be spent in investigation here.

The profile seems to suggest a couple of places ( mozilla::detail::VectorImpl::new_ , arena_t::RallocSmallOrLarge , BoyerMooreHorspool ) which spent a lot of time in Baseline. Maybe worthwhile to probe that a bit?

Screenshot from profiler which show the functions I have mentioned in the comment above.

The memset highlighted in the profile comes from reallocations done from String.unespace, which does a realloc in ExtractWellSized, which suggest that a lot of characters were escaped.

Maybe there is a fast path to be implemented here, for cases where escaped characters are more frequent than unescaped ones, but this sounds like a low priority for the moment.