Some applications aren't ended completely when you return to the home screen. If you don't end them from the list of running applications, they keep running in the background which may slow down your phone.
Step 1 of 3
Slide your finger slowly upwards starting from the bottom of the screen until the list of running applications is displayed.
Step 2 of 3
To end one running application, slide your finger upwards on the required application.
Step 3 of 3
To end all running applications, press the end all applications icon.