First of all please note that you have probably heard about hard drive drefag in the past but today technologies does not really require defrag anymore.
Two reason for that:
- Today file systems (NTFS (Microsoft), EXT3/4 (Linux), HFS+ (Apple)) use complex algorithms that constantly prevent and reduce data fragmentation.
- It depend on the user but in almost case today, hard drive oversized (there is always a lot of free space) and this help prevent from data fragmentation.
There is an archived Apple support document about that here.
What I mean is that your issue is probably not due to data fragmentation and you should look elsewhere.
Anyway if you really want to try, I'm definitively not familiar with the Mac OS environment so probably not the good person to help in that. However I have found a gorgeous and very detailed paper about data fragmentation and defragmentation dedicated for the HFS+ file system. It is available here.
I hope this can help!