With player hubs, it's always a stutter, but it also depends on the network connection - you can have someone with the best rig showing 10 FPS and someone on a laptop with 30-40 FPS at the same time.

Mass mobs combat in Karazhan is a pretty good raid tester - in raids, the FPS won't be as low as that (AVG), but the 1%/0.1% lows will be quite close. For systems with weaker and weaker CPUs, even the AVG FPS will come closer and closer as the game will get a single-core "world state" bottleneck more easily. I did some comparison benchmarks before for this, including a world boss world quest with lots of people ;)

Also, regressions in WoW are a nightmare to handle. You go to the latest raid, the FPS is fine, but someone next to you has a slideshow.