Firefox

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.

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