Starcraft II may be framerate capped. You should check this:
Setting the Framerate Cap
There are advanced settings in StarCraft II that can reduce the frames per second (fps) that a graphics card needs to render in certain areas of the game. These settings can help reduce overheating issues, but you may notice performance issues when they are enabled.
These commands should only be used as last resort. Use caution when editing your game files. Be sure to try basic troubleshooting steps (above) such as lowering graphics settings before following these steps. If you do not feel comfortable performing these commands on your own, you may contact us.
The below variables can adjusted in order to troubleshoot crashing or overheating issues:
frameratecap - This variable sets the maximum framerate in the single-player campaign or multiplayer games.
frameratecapglue - This variable sets the maximum framerate on the login screen and menu screens.
To add or edit these variables:
Click Start -> Documents and open the StarCraft II folder.
Open the variables.txt file and look for the frameratecap= and frameratecapglue= lines.
If they're there, you can set a maximum framerate by changing their value. A good starting number is 30, although you can adjust higher or lower if you choose. If you do not have those lines, add them to the bottom of the file.
Save and close the file once your changes are complete.
Close and restart StarCraft II.
To verify your framerate has been properly limited, you can view your current framerate at any time by pressing Ctrl+Alt+F, or by hovering your cursor over the Menu button.
To undo these changes at any time, simply remove these lines from the variables.txt file and save it.