A system that is more than a few years old may be having an underlying problem with its hard drive. When you rebuilt the O/S you may have only done a quick format of the hard drive which wouldn't catch serious problems with the drive. A great program to test the hard drive is SprinRite by Steve Gibson over at http://www.grc.com. When I get a new drive I run SpinRite and try to run it once or twice a year on my drives to help solve these type of problems.
Hoarding photos on your phone?
Those picture are hogging memory and could be slowing down your phone.