If the game is running slower than you think it should, lagging during firefights or stuttering at any time, there is a profiler in game that you can run.
Enter a server with other players in it and enable Settings > General > Profiling
Profiling will begin after 5 seconds and will last for 30 seconds. The game will stutter and lag quite a bit as it is measuring many aspects of the game.
While the profiler is running, move your mouse around, left and right click and press the WASD keys a lot.
When profiling has finished it will generate a profile.txt file in the C:/Users/USERNAME/AppData/Roaming/Vercidium/log/ folder. Please upload this with the other log files specified above in a bug reporting post on our Forums.
If your PC is quite old, enable both Settings > Performance > Bare Minimum and Settings > Performance > Low Quality Textures and restart the game. This will enable a completely separate, faster rendering pipeline.