Lag is a horrible, awful thing that plagues BZFlag players around the world.
How to fix lag depends on what is causing it.
Slow Internet Connection
Lag can be caused by a slow internet connection, such as dial-up. In that case, find a server that is more allowing, or get a better connection.
Satellite internet is far too laggy for BZFlag. Sorry.
Make sure Windows is not running its automatic update in the background.
Don't download something and play BZFlag at the same time.
Other programs, such as other people playing an online game on your network, can cause lag.