I'm experiencing the very similar problem, and the CPU appears to be the bottleneck, while there is no obvious reason why it should be (i7 4770K @ 4,6GHz for me). Reinstalled Win 8.1 64 last night and BF4 is currently loading, so I'll see if that does something since all the usual measures did not help at all. Thanks anyways Zorlacc for your help, but it seems there's something else causing the problem. What really bugs me is the fact, that I start out at the usual 140 - 150 fps and then it gradually slows down within 30 mins or so. Only solution: reboot the system. Something clutters up the system in the background...
Sys-Specs:
i7 4770K @ 4.6 GHz
2 x ASUS Geforce 780 GTX (SLi) with latest 344.75 Driver
16GB Corsair DDR3 RAM @ 1833MHz
MoBo ASUS Maximus Hero VII (current drivers)