The game relies on CPU single core performance indeed, but, for those situations where fps drop a lot, like in the Feast of Solstice area around the big fire, what you can do to get fps back is to turn off Vicinity NPC Names, or limit particles to Myself (raids, PvP). The same thing that happens at Feast of Solstice happens at Ethram Fal in T6 after 25%.
When I disabled NPC names at FoS, by that big fire, I went from 25 fps to 120. That’s how bad the font over NPC’s or players is for fps, and particles too, if there are too many on the screen. Another example is sieges, where, if you leave only player healthbars on (they don’t affect fps at all), with names, details and levels off, and limit particles to myself, you get much better fps. But this applies to PvE raids too, even RF. Try it and see for yourself. It’s impossible to play with the screen crowded with NPC’s or players in a fight with even 60 fps.
EDIT: I forgot to mention that disabling Shadows also gives an fps boost.