Shut down your PS4 entirely, then hold down the power button for about 7 seconds until it beeped twice. It will now start in safe mode. Connect your controller to your system with a USB cable, hit the ps button and scroll down to 'rebuild database'. Click x and let it run through the thing, it will reboot automatically. This cleans up your cache and junk on your harddrive you don't need. It doesn't delete save games or preferences or whatever. I do it at least once every two weeks to make it run smooth again.