My phone is getting slower but I don't know what I can set to not run in the background and to not run at start up. Any ideas?
Just leave the system apps alone and disable any apps you personally installed, except for the one you must have running in the background.
If there are some system install apps you don't need, disable it in Settings -> Apps.
Try disabling window animation settings in developer options (total of 3 settings). By default it's set to 1x, after disabling it apps wil pop-up almost immediately
Posted via Android Central App