iPhone 4 Responds Very Slow

Hey, you can restart the phone by press and hold the lock botton and swipe it to turn off, then press that again to start the OS.

In terms of restore or reset, if you restore the setting but not erase the data on it, then it will reboot and loaded with the default settings.

In addition to that, if you do delete everything on the phone, you can restore from backup in iTunes, and it will transfer all the data from your last backup to your phone again. (these may include apps, contacts, note, mail, browser bookmarks and more. I am not sure if songs and movies will do the same, but definitely helpful when restoring the device.)

To restore from backup, go to iTunes and under Devices section, locate you phone and right-click it, and choose Restore from Backup.

P.S I didn't have my contacts sync with my computer, so I don't know if iTunes could backup the contact database without syncing them.

I haven't rollback to previous OS before, because they all work fine to me. However, if you want to downgrade the OS, you can try the following:

1, download a iOS firmware file of the version that you want to use (search in Google and find suitable ones)
2, make sure it is the right one for your device and your prefer version.
3, in the summary page of your iPhone on iTunes, press Shift and click restore, this will allow you choose a specific file that you want to use
4, locate the firmware you've downloaded and choose it, then iTunes will use that for restoring your phone, instead of downloading the latest version.





Now, don't assume this would work because to my knowledge iTunes needs to validate your device and the server before it can be fully restored. Apple could(and already) disable the validation process for some earlier versions of iOS software when you try to install it.

If this doesn't work, I think you'll have to use 4.2 and check if any apps cause your problem. And if you are still not happy with the result, just bring your iPhone to the Apple Store and ask the Genius if they could solve it.

Good luck!