When I change the video settings, the game's performance slows down. What can I do?

After changing the video settings using the in-game pause menu, the game's performance may drop.

This is because the game is rebuilding the level using the new settings at the same time as you are playing it. However, performance should recover within a few minutes. If it does not, quit the game and close the application. When you re-launch the application, the new video settings will be in effect straight away and so performance should be back to normal.