LastPass is pretty bad as well if you happen to have that installed. Look for onloadwff.js.
Protip: if you use Chrome, you can click on the profile icon and choose "Open Guest Window," which won't have any extensions. Good for performance analyses.
I admit I'm not sure about Firefox. I'd recommend Private Browsing, but it has slightly different functional behavior from non-private mode (e.g. IndexedDB is blocked). Also it's possible to enable extensions in Private Browsing, so you can still trick yourself.
Relevant: analysis of top ten Chrome extensions and their impact on page performance. Grammarly adds 100ms on average to First Input Delay. https://github.com/treosh/exthouse/blob/master/README.md#analysis-of-top-10-extensions-from-chrome-web-store
@nolan Coincidentally, about a year or so ago I basically stopped using browser extensions. I have just one in each of Chrome and Firefox. I don't know why. :\ I think I just had to reinstall everything with a new computer, and didn't sync extensions, and then after a few months realized I didn't need any of them anymore. (HTTSPAnywhere is the only thing I run now.)
This is a private mastodon server for members of the Team McNamara Group.