The one and only reason I stopped using Vivaldi every time I tried it is the performance. But it becomes more and more obvious the developers simply have different priorities.
Then again, I also use uBlock Origin... perhaps Vivaldi is slow without it. I'll also admit that theoretically I'd expect Vivaldi's UI to be slower due to being based on web technologies instead of native, but I can't say I can perceive a difference except in edge cases.
Tab handling was at least x1000 slower than Chrome last time I tested. It got dramatically slower in the last few snapshots and the official 3.6 Stable build.
Its not just tabs, address bar handling is totally broken, opening new tab and starting to type quickly will result in eaten letters. Opening new tab, clicking CTRL-F and starting to type will again open with ~1 second delay and lost letters.
Then we get into broken javascript causality https://jsfiddle.net/rasz_pl/v36nozsw/ open in normal browser and Vivaldi, Vivaldi address bar handling code is overriding scrollTop with delayed location.hash update.
Seems to me there's a subset of power users on Firefox just waiting for any other browser to implement tree style tabs.