It's not a matter of speed, it's a matter of preventing the app from losing state, background tasks, timers, notifications etc. whenever you do something as nor

It's not a matter of speed, it's a matter of preventing the app from losing state, background tasks, timers, notifications etc. whenever you do something as normal as take a call and browse the web before trying to go back to the app at hand. If it can't handle config changes it will crash a lot, sometimes even when not fully backgrounded. That being said there are plenty of apps (video players for instance) that disable config changes on rotate as to more quickly switch between orientations. They still need to handle config changes properly for times like when the app is put into the background, and sometime later restored in a different orientation.