There are several different performance problems in Legendary Edition, gathered from my own experience or other topics on this board:
Citadel Presidium reflections are rendered at 30 FPS.
The "bitter spot" on uncharted worlds, when there is long line of sight from one site of the map to the other, while both CPU and GPU usage are very low.
Framedrops after rapid camera movements. Staying in a spot on an uncharted world and calmly moving the camera around with stable framerate, but one slightly faster movement and stuttering occurs along with huge framedrops, likely from 100+ down to 40 or even 30.
Other strange behaviours, like some people are stuck at 30 frames with potent hardware all the time, or peripheral hardware plugged in causing constant stuttering and poor overall performance.
Quests like Garrus mission, after killing Dr. Heart aka Dr. Saleon the framerate drops and never recovers, even after returning a few hours later and docking again, the frames are still very low on that freighter. Or killing the indoctrinated Salarians on Virmire causing a huge framedrop to less than 20 FPS, at least the framerate is getting back to normal after talking to Rana Thanoptis.
There is some work to do for Bioware, I hope they get it done.