Go to solo, press Ctrl-B, watch the bandwidth monitor. If you don't trust it, open the Windows bandwidth monitor.
What can be influenced by other players (if you're not in solo) is rubberbanding and, in general, the number of ships (including NPCs) in your instance. And yes, with >30 ships in my instance (3 or so players, each bringing 10 NPC ships into a busy station - Jaques for example) inside a station my FPS will drop.