Well, as previously noted, it is apparently possible to root without leaving any traces of doing so. You could then revert to stock firmware in the event you needed to return the phone for any reason. Obviously, if you're not bothered about messing around in this manner, it is not compulsory!
Truth be told, I'm surprised you seem to be encountering 'slowdown' problems with your phone as I've not really heard many reports of users having such issues with modern devices. I'd imagine you're on Android 4.3 or higher? Should ensure that TRIM support is there and isn't causing any slowdown.
An alternative cause could be rogue apps but, again, assuming you are using up to date versions of apps which are decently coded, this shouldn't be an issue either. You could always try uninstalling then reinstalling the apps. I'd imagine this would have the effect of 'wiping' the cache and dalvik cache for each of the apps in question, rather than just wiping the whole phone.