So, Halo PC and CE have this thing fo running the game in a window. It makes framerate better unlike fullscreen which is ok. In the command line parameter you have to type -window.

In H2V you need a lot of requirements to play the game. Many people have problems with H2V cause of the requirements and it runs slow on the pc. There is a way to make framerate better for H2V by typing -windowed in the commandline parameter in the preperties window. It helped me out cause I have very low RAM. I have 446 mb RAM which is slow. Windowed H2v is good but bad in some ways. You can have less FOV which is Field of View.
Trust me this can make h2v a little faster................(I'm so sad my game is incredibly slow)