`fit-textarea` causing major performance issue when typing in PR description textarea with Chrome #4275

With the fit-textarea feature enabled, I'm experiencing a severe performance degradation when typing in a PR description textarea. Each letter appears to be taking ~500ms to be handled, making the feature unusable as it takes multiple seconds for a few words to appear.

Performance profile shows this line taking 1.5s per call:

Disabling the fit-textarea feature removes the delay.

I've uploaded the Chrome performance profile json file here: https://gist.githubusercontent.com/with-heart/f0d3b0914d2c25e6f635d82f91404e10/raw/af6e016d11ddf07e73a4459a6f40b1cc4ad58885/refined-github-performance-profile.json

This issue appears to be related to #4266.