On some sites, the CPU usage goes to 100% (one core) and it freezes any activity inside the browser. For example I can move the window, but not click the tabs or in the page, and if I do, the actions are delayed.
One site in particular, izismile.com, can make this 100% usage to go for tens of seconds, even a minute or so. The network activity doesn't seem high, a few KB (LAN included), so I thought maybe it struggles with communicating with relays, but sometimes it is possible to have a few seconds break, during which if I open up other sites, tor.se for example, they open up fast, no glitches.
about:config changes, all coming from privacytools.io. Also uBlock Origin, the only foreign thing added.