For the quest in Legion: Remix titled “The Only Choice We Can Make” located in Northwest Suramar, just northeast of the path that leads to Azsuna.
This quest has you Incinerate the Shaladrassil Root.
Upon walking up to the area and it being loaded, I experienced such an extreme frame drop that the game crashed and I got a message saying my graphics card is not supported. This did not stop the game from opening, so I did and then set my graphics to the lowest settings and logged in.
To resolve this, I positioned my camera to look at the floor, zoomed into first-person pov, and then followed my map to the quest, accepted it, and continued to the root to burn it.
Once the root was burned and the objective completed, ALL THE LAG VANISHED. As long as the root wasn’t in view on screen, I was able to walk with minimal lag. As soon as I got close to it, insane lag. I had to use the item from the Quest dropdown menu to burn the root. Luckily it worked.
So the object that is the “Shaladrassil Root” in that quest is a problem. Once it was removed, everything is totally fine.
Not everyone is so analytical though, so a fix is welcome.