I don't get this. 3 comments (so far) from people claiming slowness. I've been using Vivaldi for my "time wasting" browser (Hacker News, Reddit, Techmeme etc.) for quite some time and never see any slowness. I launch it dozens of times each day. I have a 2+ year old i7-9750H with 16 GB of RAM and I always have Chrome with 4 tabs min, and Firefox with 2 tabs min open simultaneously with Vivaldi.
Perception is reality. One commenter above felt that the "2-3 seconds" it took Vivaldi to start up was intolerable. Every post about this browser is always flooded by people whose main benchmark is that if a browser does not match Chrome on subjective performance, it's shit. I don't get it either.
For me, tab tiling is an absolute killer app, unique to Vivaldi, and the somewhat slower UI performance than Chrome is well worth it just for that.
I think the people complaining about speed use new windows instead of tabs. I love Vivaldi but new windows take that 2-3 second load time meaning I don't even consider opening new windows outside very specific situations. Tab tiling alleviates this but then I have to remember different focus/bind schemes from my window manager.