Maybe you are referring to a system restore, not to a fresh installation. These are two different things.
A fresh installation usually involves the formatting of HDD, and this certainly will wipe everything on the disk.
A system restore will not speed-up the system, because it simply restores the original files of the operative system, but it will not remove all the registry mess, so basically it is an useless procedure. The main goal of a system restore is to recover a not booting system, but it will not make the system faster