What's so wrong about that? All the heavy browser lifting is done by the browser engine, which is native, and everything else is just... chrome. Which doesn't really have huge performance requirements.
UI is very latency sensitive, even a couple of frames of delay can be detrimental to the feel of a UI.
Comparing the delay of the UI with many tabs (30+) Vivaldi wins easily over Firefox and Chrome.
I did not say Vivaldi was slow, just that performance is relevant in UI. I'll give Vivaldi a try.