That's definitely a bug. Hmm.. I don't know why it would be. Which OS? I would be happy if you could profile the javascript to see what is up with that, but should be on a dev build. Open an issue at https://github.com/emnh/rts-reasonml/issues so we can communicate on it if you are interested. A dev build is at http://emh.lart.no/publish/rts-reasonml-dev1/ .
...and ~50fps on the integrated graphics of i7 mobile 8th gen (oh, and on battery, with power saving features activated). WebGL performance is a weird beast.
Restart the whole browser. You probably had the GPU drivers updated while Chrome was running and now it's running in software rendering mode. Check my other comment in this thread for more info. As a WebGL developer I encounter this problem a bit too frequently.
I think this is the answer. I did do a driver update and now, after restart, it works fine! Ugh, what an annoying bug!