I know, running slow is a bit of a paradox.
If, however, your computer does not automatically and regularly defragment your system, computing may become slower and you may be left with a PC or laptop that always hangs... or takes you back to about five years ago when every program takes decades to load. Haha.
Also note that not all registry cleaners are created the same, and some may actually be slowing you down further. So, check and compare reviews before committing your precious PC to one.