I'm on PC, decently powerful (100+ FPS with an Ultrawide resolution). I read that maybe crossplay could be an issue, so I disabled that, and just did a Legion event by myself, but it was still horrible.
It's mainly the Legion events, since they're the most frequent for me (World Tier 2). I barely see any other players in the world, but occasionally in one of the Vampire zones, where it's fine. World Bosses have been alright, but I've not done as much of them.