Netflix is the main example. Takes minutes to load the page, usually stalling on a TLS connection to assets.nflxext.com
If I disable IPv6 in about:config, it loads instantly. This happened like a year ago due to some MTU issue and was resolved with some changes on Sonic's side.
Another example site:
slot1-images.wikia.nocookie.net which is loaded when visiting this page for example: https://hyperioncantos.fandom.com/wiki/Meina_Gladstone
For whatever reason, Chrome doesn't seem to be as affected.