Shaders play an important role in graphical performance. We do not know what they changed, asides from it looking different the code may also have been optimized further to enhance performance. Beyond that, performance is not all too bad. In many areas a 100+ fps is not impossible, and an fps of approx 50 - 60 in warzones is also not out of the question; on highest settings at 1080p.
FPS doesnt necessarily mean performance issues. I have around 110 FPS while alone and still get like 60-70 in a raid setting. Some things will bring my machine to a crawl. Other things will make me hitch/stutter or lag spike. Other things give graphical issues. Other things have UI problems/load screen problems/long load times/screwed up things when you shut the game down/memmory leaks/ability lag or no fire or registered fire with nothing happening or ability no fire with a cooldown anyways.... all those things have nothing to do with FPS and everything to do with performance.