Why does my phone keep freezing?

my phone keeps freezing and lagging all the time like maybe once every minute or two...sometimes for 10 or 15 seconds sometimes for a minute or two. It has been doing since I got the phone the other day it is starting to drive me crazy ...I updated it and at first it seemed to do the trick but the next day after updating it starting freezing up again I haven't downloaded anything expect a couple apps I had on my old phone like youtube ...does anyone know whats going on