This may be old news to some, but maybe it will help a wayward soul somewhere....

Vivaldi was really slow when starting up, and it would stay slow with multiple cores pegged at 100% on my Linux system. Eventually it would crash and I'd have to start it back up again.

Slow in this case means delays in responses to clicks, scrolls, etc.

Anyhow, I discovered that scanning pages for RSS feeds was enabled. I disabled that and my browser starts up very quickly now.

If you have a lot of tabs and RSS scanning is enabled I believe it tries to load every page and scan the contents, but it was too much for my fairly beefy system.

tl;dr: disable scanning for RSS feeds if you don't use it.