The slow down issue is due to the root. Unless your using software like boot screen editors (which can be done unrooted), theme chooser, memory searchers for apps like GameCIH... there's honestly no reason to be rooted.
if the kind of reset you want to do is just taking it back to how it was when it was fast and no apps really installed from play store, then just do a normal reset through your custom recovery. I'm guessing you have a custom recovery if your rooted. if you don't know how to get to recovery, google your phone and how to get to it. each phone is different
if you don't have custom recovery, then just do it from the settings. You shouldn't lose root. IF you do loose the SuperUser application though in the reset, get a custom recovery and just use google to find a standard Super User zip file to flash from your custom recovery. IF you don't have the custom recovery, your going to need to either get it to be rooted or go through the whole root process again if you want to be rooted.