There's a handful of horrible websites that use enormous amounts of CPU and memory. You don't need too many of those to drag a browser down. I am a thousands-of-tabs user (and yes it's an awesome productivity hack for a number of my workflows, but never mind that) and I don't normally have any problems. Unless I make the mistake of opening some trash website (Slack used to be like that, I've not used Slack in years so I don't know if it still is) and then it bogs down everything.