There's CPU bottleneck, but there's also intense GPU requirements for high settings in some scenarios. At least according to task manager GPU was loaded at 100%, so I suppose, it was GPU bottleneck.

It is when you're in a raid and there are dozens of spells going off all around you. They have however made some tweaks recently, but it's still not incredibly well optimized.

Sounds like particle system batching and/or overdraw issue. Probably this could be solved by more intelligent batching and culling, or by throttling particle count/resolution when the scene gets too busy.

You've solved it!