Make sure your video card meets the system requirements. If it does, and you're still having problems, you may want to email the place where you bought the game from (if it's a download). It may also help to use different video drivers (possibly newer, possibly older depending on the age and model of your video card).
If you have an antivirus, you may want to disconnect from the Internet and temporarily disable your antivirus. Since modern antivirus scan in real time, it could be scanning everything the game is doing, which will slow it down. You may be able to add the game exe to the antivirus as an "exception" and have that work without disabling the whole antivirus.
If you continue to have problems, you might want to email the place you bought the game from (if it's a download) or email the developer or publisher about the problem. Be sure to send them your computer's specs including the name and model of your video card (e.g. - Geforce 8600GT with 256MB of video RAM). You can find most of this information by running dxdiag, or a tool like speccy.http://www.piriform.com/speccy