Try the same test with Vivaldi 3.7, 3.8 or 4.0... They've optimized the browser interactions quite dramatically so it's much faster than the video shows. But... It's still slower than Chromium's UI.

Vivaldi UI got significantly faster exactly two times from product inception. First time was a random Taiwanese hacker reverse engineering it, noticing they were resizing _all background tabs_ every time you resized browser window, and sending them a patch https://github.com/WillyYu/vivaldi_1.7. The second time Vivaldi themselves speed up Tab open operation, and only this one operation. "tabs open twice as fast. increasing speed up to 2x" sounds impressive until you measure 400-500ms instead of previous 700-900ms, versus ~16ms in Chrome (one screen refresh).

You're absolutely right, the painting, or whatever you call that, of the UI elements of Vivaldi itself are definitely sluggish.

I was thinking of the normal in-page stuff. My apologies if I misled anyone.