The only reason I switched away from Vivaldi was the sluggish UI (too many things on the main thread, I guess), but I hear there have been improvements in that area. I may give it another try.

qutebrowser on the other hand seems to have limited support for addons and the builtin features are good, but not on par with what addons can give you (https://github.com/qutebrowser/qutebrowser/issues/28).