The most likely cause of lag for Arma 2 is the server needs a restart, it is reccomended to have a restart every 3/4 hours depending on the amount of active players on your server.
You can easily set automatic restarts by:
- Cliking on the scheduled tasks button within the game panel
- Click new and select scheduled restart
- Set the name as restarts
- Select Daily and choose a start time / date
- Click repeat task
- Choose how often you want the restarts EG. (Repeat every 4 hours)
- Select "For 24 hours"