Showing the ScenarioObjectiveTracker frame in delves causes a huge loss in performance. On my system it’s in the order of 20fps.
The frame I’m talking about here is the sub frame of the objective tracker showing the current delves affixes/lives/objective.
If you’re having performance issues in Delves try showing the FPS counter and then collapse your objective tracker as a test.
(A similar mini frame is used in M+ dungeons for the timer etc. This too could be a framerate killer but most people use addons that hide it in M+.)