The problem with fps. (World of Warcraft)

Start with PC:
Graphics card: gtx 960 4gb
Processor: i5 4690k
Op: 16 gb
The problem is that in the usual location I have an average of 350-400 fps, with or without mobs, and when I go to the capital, for example Stormwind, the fps drops to 70-90. How can this be fixed ?