There are a few possible causes of FPS drops in fgunz, including:
Server lag: If the server you're playing on is experiencing lag, it can cause your FPS to drop, even if your own computer is powerful enough to run the game smoothly.
Network latency: If your internet connection is slow or unstable, it can also cause FPS drops.
Hardware issues: If your computer's hardware is not up to the task of running fgunz, you may experience FPS drops
In-game settings: Certain in-game settings, such as the resolution and graphics quality, can also affect your FPS.