When you move the Mipmap Levels slider in the Video Settings, Minecraft reloads all its textures and sounds immediately. This makes it very hard to change the level to what you want, the game is frozen until everything finishes reloading. If you try to drag the slider from one end to the other, it stops at each level to reload everything for some time.

This problem is made much worse by large texture packs or mods with many textures and sounds, the delay for moving the slider can be well over 30 seconds.

Ideally, the textures and sounds should only be reloaded when you click "Done" from the Video Settings screen. This way players can move the control freely, and apply the new setting only once.