Huge lag issue while switching Mipmap levels

In the old days, it used to be quite smooth switching Mipmap levels. Then it got applied after you exited the main menu. Now it switches instantly, making changing Mipmap levels quite annoying.

Here is a video, that shows how annoying it is to switch the Mipmap levels in a flat world.

(I was unable to show the diffrence, due a 10 MB attachment limit)