I'm curious what area of Vivaldi is perceived as slow. I use Vivaldi at home (on a 9-year-old desktop), and Firefox at work, and the only things that come to mind are tab opening/closing could be better, and some very CSS-heavy animations such as the NYT's map of coronavirus's spread from Wuhan are much better on Gecko than Blink. But day to day, I have no complaints about performance from either.

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.