I don't know if this is a Catalina issue, or when I first started noticing it, but after a period of time or with enough tabs open Safari will start to open and switch tabs much slower, it will hang for a second and even beach ball.
First noticed this on 13, and now also on 16. It's very annoying, but can clear for a while if I close safari. Of course usually need to restore tabs so I can continue where I was. It shouldn't be a problem with too many tabs, as was certainly using this sort of setup on systems with 8GB ram in 2016 without problems. Just using one extension, 1Password.