Seems like there is a massive issue with FPS staying consistent on multiple configurations including some high-end machines.

Seen on the official WoW: Forever Beta Discussion forums Many players have complained about stable performance. Blizzard has yet to respond and that kind of worries me, they have responded to other bugs as far as gameplay mechanics go and some server side issues but the thread in question has 89 replies, 14.5K views and has recent activity. at the time of posting this.

Hopefully Blizzard does address this issue and isolate what might be causing it, but for it to happen across multiple configurations is concerning to say the least.

So from what I gather, there's a memory leak that causes it. And the issues only start when you enter a major city, even if it's not too crowded compared to some other areas. Outside of those, you get smooth FPS. But after you visit a city, get the drop and leave, low FPS remains. Mine drops to about 25 FPS whenever I'm in Ironforge.

To solve it, you've got to log out and log back in again. Won't help you if you're still inside a city, but outside of them, it should be good.