How to speed up your cellphone

My girlfriend had some troubles with her cellphone. It took a long time (2 seconds delay) to go back from the menu to the main screen. I could not find any configuration problems, but finally found that the picture she used as a background image delayed going back to the main screen. It needed to load every time she returned there and that took some time. Removing the background image was the solution and now it takes almost no time to go there.