Used to be that way for me too, but I believe with the latest LTS the slowdowns just went away. I'm using Firefox through the default Snap packaging every day, and it's just fine.
This is also with the distro installed with all default choices. If memory serves me right, using btrfs for one can result in any number of weird performance issues.
I thought it was fine too until reading this thread. and I tried opening some cpu intensive pages in the snap version and in a standalone version. the Snap version makes my cpu max out at 100% and i get soft frozen out of my desktop, the standalone never went above 80% .. Now we can blame the pages for being shit in the first place but, I dont want to have to restart because I opened the wrong page.
This is also with the distro installed with all default choices. If memory serves me right, using btrfs for one can result in any number of weird performance issues.