Server lag and latency can affect the response time for actions, but it doesn't automatically translate into affecting your FPS.
FPS is determined by the raw processing power and data transfer speeds in getting the data rendered on your screen. Remember, you monitor has a refresh rate, in most cases 60hz or better. It has to keep up with 1920 x 1080 pixels tagged for 32bit color data per pixel, plus Z buffer, transparency data and such... and your card also needs a buffer flip for the changes in the pending frame as well (there's more, but you get the idea). So, you card has to continuously send all that data to your screen so it is displaying something at all times, or it goes black. Even when NOTHING is going on, and no network data is being sent/received... you still have an FPS rating because yor card is refreshing your screen. Pull it up under configuration while on the Title Screen for a perfect example. The one that has Start, Opening movie, Configuration, Exit. The one that loops the FF anthem and if you let it sit long enough without starting the game it goes into "attract" mode and starts playing movies after a while. You may see it running 150+ FPS there if your VSYNCH is unlocked. It's not relying on the server for much of anything at that point, and there is little if anything to process either--so it's running full speed ahead. The same thing happens when you first zone and your screen is basically black with the loading notice.
Now, when your system doesn't have truly dedicated networking hardware and such (don't find many with NPU's anymore), then heavy network traffic can have an impact in a round-a-bout way because it is stealing processor time and such. This applies when you have 350 users huddling around the crystal in a Mongolian Clusterf*ck situation and your system is having to process all that extra character and custom texture data being pumped in through your network card. But the bogging down in that situation is because it is consuming local resources to process game data (CPU cycles, Memory bandwidth) that the system might otherwise use just for rendering. It is NOT because you are waiting on a response from the server to tell it when to render something on the screen... it's your system is bottlenecking somewhere because of the data overload in general.