How can i fix my FPS

The game is from 2004 and it's not optimized. A LOT of lower end pc's should be able to play this game at 200 fps which you can! Only on singleplayer but on multiplayer, you get like 20-60 fps. All the addons, people, map, the game was never optimized for all this. The Dev's said it themselves. That's why they're making a new game called Sandbox but they never know if they'll release it.