I found a bug that slowed it down a great deal. It was the fact that they put multiple `filter` styles onto the tabs, even when those filters did nothing useful. It was still extremely taxing on the GPU driver.
But, since you can customize the UI with your own CSS (there's a hidden setting you need to enable first, in vivaldi://experiments), I put a simple `* {filter: none !important}` in there. After I did, it felt as if I bought a new computer.
The fun part? They still haven't acknowledged it. Even after I gave them the exact steps to fix it.
I just did some tests and was unable to replicate your results. I'm on an i5 2019 Mac, and Vivaldi takes about 200ms to open a tab, with or without CSS filters. (200ms is bearable, but it is slower than I would like.) I have Hardware Acceleration on and Use Animation off.
I'm not sure about the internals, but when those filters are there, the GPU process causes a lot of CPU load on every frame. So much that it often hits 100% of the single core it uses before it's able to draw at 60 fps. Oh and it redraws the tab bar a lot. It redraws it every time you scroll a page, and it continuously redraws it when there's a tab playing audio in order to animate those waves emanating from the speaker icon. Yes, it redraws the entire tab bar for a 16x16 animated icon.
They unfortunately don't have a public bug tracker.
But, since you can customize the UI with your own CSS (there's a hidden setting you need to enable first, in vivaldi://experiments), I put a simple `* {filter: none !important}` in there. After I did, it felt as if I bought a new computer.
The fun part? They still haven't acknowledged it. Even after I gave them the exact steps to fix it.