Firefox

If you like to manually write custom CSS rules for every website that you visit, you could use Stylus. That would be fast.

Additionally, websites that use Bootstrap >=5.3 actually do have adaptive dark/light mode that gets set automatically as long as your browser communicates that to the website in question. So this isn't universal, or even well adapted, but this is also fast.

Dark Reader is what I use. I rarely notice it slowing down page load times, and it hasn't been enough to bother me so far. The thing about it that does bother me is the rare occasion when specific websites look weird with it, but I can just turn it off for those specific sites and it's fine.

I only use Dark Reader on my mobile (Pixel 7), and I don't really notice slowdowns enough that they bother me.

You can set different types of filters on Dark Reader. The default, Dynamic, tends to be the slower, but it gives you a better experience. Static is faster, but with a more blunt approach.

https://darkreader.org/help/en/#theme-generation-modes

https://addons.mozilla.org/en-US/firefox/addon/dark-background-light-text/?utm_source=addons.mozilla.org&utm_medium=referral&utm_content=search

I use dark reader but people say this one has better performance.

Downvote target comment, however light mode has been studied to be clearly superior to dark mode - so I just use that

Benefit of that is that there are very few websites that have only dark mode available, while most content that has the dark mode option looks more consistent with only white mode on