STR:

1. Load https://comfortable-weeder.glitch.me/ in side-by-side Firefox vs. Chrome
2. See how long it takes the animation to complete (filling out the whole color diamond) in each browser.

ACTUAL RESULTS:
Chrome takes ~10 seconds.
Firefox takes ~50 seconds.

EXPECTED RESULTS: we should be on par.

Here's a profile of Nightly for the first ~15 seconds of this load (during which time Chrome easily finishes the whole thing):
https://perfht.ml/30r8956

We're spending 50% of our time in the JS engine calling fdlibm::sqrt (called by fdlibm::hypot), as shown in this invert-stack view:
https://perfht.ml/30mqWi1

Note: this demo's author (Tab Atkins) is currently messing with it a bit, so it doesn't load anything interesting at all right now. He promises not to make it faster, though :) (per twitter thread at https://twitter.com/CodingExon/status/1129145052708519936 )

Anyway, hopefully it'll be back up and testable in time for whenever someone can take a look at this.

The problem is that the rasterize function is an async function, and that SpiderMonkey does not yet compile Generator / Async-functions with its optimizing JIT.

Moving the inner loop which loops over x in a separated function should improve by optimizing this inner function with the optimizing JIT and relying on the ecmaHypot function.

This demo seems to run better these days. Daniel, what do you think?
https://share.firefox.dev/3WoZInu

Indeed it does! I just tested and Firefox seemed to be slightly faster than Chrome, actually. I'm guessing the "SpiderMonkey does not yet..." note from comment 2 has since been addressed.

I ran mozregression to find the fix range. In old builds (e.g. comment 0's datestamp) I'm not seeing anything as bad as the 50-second time that I mentioned in comment 0 -- I'm seeing something like 5-6 seconds in newer builds, maybe 6-7 seconds in Chrome, vs. 8-12 seconds in older Firefox builds (with noticeably smaller progression per frame towards the end).

But that was still enough to find a fix range:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=b47dc08fc77ad2f26cee89a216c333ff490d195f&tochange=c6d819bd39daf86acd5e0c5bce5d74c2db8b8d64

That includes bug 1682623 "Enable Warp Compiled Generators and Async Functions", which sounds like it'd be what fixed this, based on comment 2.