Every time I look at a CPU profile of a website and I forget that I have an extension installed, I'm reminded of just how awful extensions are for performance.

As an example, here's a trace I was looking at of Pinafore, and… oh my, what is this gigantic JavaScript file that's taking 870ms to parse and execute? I thought most of my scripts were pretty small?? Oh right, it's the Axe extension deque.com/axe/

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. github.com/treosh/exthouse/blo

This is why I'm horrified when I see browser windows with a dozen extension icons. You're just begging your browser to be slow.

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@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.)

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