Just my curiosity getting the best of me on this one; why would FPS, in any game, cap even if any of the following: HD/CPU/GPU/RAM/VRAM aren't even close to being saturated?
Take for example BF3 & WoW; BF3 has my 4870s capped at 100% @ 1080p with a High+ setting (tweaked between High & Ultra) which would be a clear indicator that my cards are the bottleneck as my CPU/RAM have a lot of idle/room left... and I'm sure if I had an Atom with a 7970 (obviously exagerating!), the CPU would be tappered and the GPU at less than 40%!
WoW on the other hand, no matter the graphic settings, will NEVER cap ANY of the components; like at MOST my CPU reaches 40%, GPU ~75%, VRAM ~1.2GB / 2GB (so I'm guessing 600MB per card).
I'm monitoring using Radeon Pro, GPU-Z, W7 resource monitor, W7 CPU meter (the gadget) etc... and they all indicate a trend where WoW just can't use all resources no matter what. I game on a 120Hz monitor, so all my settings are set to 120Hz and vsync is off (not like the 4870s can cap 120FPS at all times, but depending on the scene, 100 is the average in most games) It's as if even if I had an octo-core with a quad SLI setup, I'd get 200FPS BUT nothing would be capped in resources...
KoA capped my cards, Just Cause 2 caps my cards, etc...
The only explanation is that the WoW engine doesn't scale very well? I'm not complaining, it's just something I found interesting while monitoring & benchmarking for a friend to compare... hell, WoW is almost 8 years old now.
MMOs have a lot going on server-side. That's why you could have 500 NPCs on your screen and have silky smooth framerates (if your rig can handle it) but hang out in an area with 500 players and your framerates will be in the toilet (or some other undesireable place).
The bottleneck (so to speak) has nothing to do with your computer