Hi, just wondering if you have an SD card installed? If so what might be happening is after you do a factory reinstall you may have viruses from the SD card loading up and freezing your phone up. I had 7 for a long time, then I realized what was happening. So what I did was download malwarebytes free from the play store and ran full scan then delete the apps and files. It was like night and day, no lagging and very fast.