I'm using an S-Video cable to plug my laptop into my TV.
For some games, the speed is fine. Others are drastically reduced or increased.
What am I supposed to know (refresh rates, laptop processing speeds and the like) so I can set this up to optimal performance?
Speed issues.
In terms of the actual signal sent to the TV, you don't get a lot of choice in refresh rates or resolutions. If those limitations are visible to applications (rather than having the video card / drivers present a VGA-like device and downsample for the TV), some might have trouble dealing with a frame rate that's half of what the developers expected; a VGA-resolution image on a TV can only be updated at 25 or 30 frames per second, whereas it would be at least 60 on a VGA monitor.