24 bit often need to be stored as 32bit value so many drivers just offer 32 bit as option and not 24 bit, even if 8 bit are completely wasted (we are speaking about the color depth of the display not of a texture, displays don't have transparency

).
When using OpenGL or DirectX in window mode then the game must use the color depth of the current video mode, it has no choice. The only way to change the color depth would be to change the color depth of the desktop or go into fullscreen mode where it can do whatever it wants to do. I think the best thing is to open a webpage when the game is running slow, and on that webpage we give tips&tricks for speeding up the game.