The same thing happens to me in the servers, low fps, especially in king of the hill, but the PC is not being demanded in CPU or GPU, must see a bottleneck in the same network code. I support a change in the network system separate from the overall performance (asynchronous), if applicable.
Here are my system specifications:
CPU: i7-3770k
MB: P8B75-V
GPU: GTX 1060 6GB
installed on SSD
FP: CX650M
Monitor: 1080p