I just ran a few quick tests with the Top 50 extensions and an NYT article. The charts are ordered by largest median metric value.
https://gist.github.com/mattzeunert/de3c8aedd2936a34eeb88b62...
The Largest Contentful Paint (LCP) chart suggests an interesting phenomenon. Extensions like Dashlane and Evernote appear to slightly increase when that paint happens. These extensions load a large amount of code, but don't block the initial render.
Grammarly does not seem to push up LCP much. Its JS code runs before the page starts rendering, so maybe it is less likely to compete with other code later on.
Normally running code before the page renders would be bad, but if the page is network-constrained at that point it might soften the impact.
I recently switched from LastPass to 1Password because of the added latency from Lastpass. Lastpass adds about 70ms to first contentful paint on example.com. 1Password, on the other hand, runs after the painting is done so it doesn't block rendering. I polished up a blog draft I had lying around about switching to 1Password: https://joe.schafer.dev/passing-lastpass/
> I'd be much more interested to see how extensions like Evernote or LastPass increase the time it takes for a real webpage (e.g. "nytimes.com") to finish painting the viewport not including ads.
I reinstalled Lastpass to test on nytimes.com. It takes 58ms to evaluate onloadwff.js (the Lastpass entry point) before any content is rendered.
The only annoying thing is that LastPass requires the whole page to reload first - I don’t know why Chrome can’t load an extension into an already-loaded page.
I found what I was looking for, this is the article I ran the tests on: https://www.wcpo.com/news/education/higher-education/miami-u...
This can be very noticeable though, leading to sluggishness, stuttering scrolling, slowing down other tasks, increasing fan noise on laptops. uBO in my anecdotal experience may almost make a bigger positive impact in this fashion than simply at the initial render.
The main takeaways seem to be that 1) extensions in general wind up massively increasing CPU time (by 5-20x) when loading "example.com", and 2) ad blockers wind up massively reducing CPU time (by 4-20x) when loading a "WCPO news article".
Which makes me happy that I use uBlock (edit: Origin, thanks below), and sad that I have to use LastPass.
HOWEVER -- I feel like both these metrics are potentially highly misleading, because CPU time isn't something the user directly observes -- it might be the limiting factor, or it might not affect the user experience at all (because the CPU usage is happening while waiting for further network resources that are even slower, or the CPU usage is happening after the page has visually finished loading all relevant content).
I'd be much more interested to see how extensions like Evernote or LastPass increase the time it takes for a real webpage (e.g. "nytimes.com") to finish painting the viewport not including ads, and similarly whether adblocking actually decreases the same -- or if all the advertising stuff really only happens afterwards. (Because sites are architected differently, you'd need to compute an aggregate score across a range of common sites.)