Actually not the registry is defragmented.
The registry is kinda like a virtual thing.
There are several files that provide the entries in the registry.
So basically there is not such a thing as registry defrag at all.
And about defragging:
Q: Will it speed up things?
A: Yes it will.
Q: Will I notice it?
A: No you won't unless you are using a FAT file system or if you use NTFS and the drive is really full.
Q: Is 301 km/h faster than 300 km/h?
A: Yes it sure is.
Q: Will you notice the difference between driving those two speeds?
A: No you won't.
Ok, I admit it.
You could notice it if you have to travel let's say a light-year through space