Under Game-> Camera options, disable: "Use Reflex Context Guide" and "Display Contextual Action Prompts", should help with problems at joint attacks with bosses.
It's not ideal, since you're new player, because you need to know when to CC, but it helps.
Though I really think there's some memory leak with UI since Awaken patch, because when i hover in UI over item or Transmute tab, fps drops from 100+ fps to low 30 or less.
Move mouse away or close inventory and fps back to normal. Happens everytime, tho I never bothered to report it under bugs section. Wonder if it's only me?
Also just closing map gives 10+ fps, game's UI needs serious bug-checking and optimization.