I'm going to get an SSD, making that my boot drive and moving the existing hard drive to the optical drive bay. I will keep my data on the original hard drive.
MacPerformanceGuide.com says "If the Boot drive is a fast SSD, moving the home directory can be self-defeating for performance."
Let me be clear. I'm talking about the home directory specifically. MacPerformanceGuide talks about the goodness of moving the Documents, iTunes library, etc. folders, but not so the home folder.
Why is that?
We are not talking about wearing out the SSD with excessive write cycles; we're talking performance.
Best Answer
OS X stores a considerable amount of data, from program caches to a multitude of preferences and other vital tmp files, in your home folder (Library to be precise). Shifting these off the SSD is detrimental. The system would still pull files from your speedy SSD but any cache files from your apps (both third party and default) would be pulled from traditional and cumbersome HDD.
Offloading large, static files, like music, movies, books, and other documents that benefit little from the speed of an SSD are recommended. But moving small core files that benefit greatly from solid state disks isn't. It's crucial to keep as many integral system components on your SSD as possible. A 2 GB movie or 1,000 songs won't benefit because movies don't need to be played faster and neither do songs. They may load a little quicker, and in the future, when SSDs will have TB of space, this won't be a problem anymore, but for now, it's all about triage.
I suppose you could try to micromanage this aspect but to be 100% safe, I would just move the big ticket items. An 120 GB SSD with 5 GB on it won't be faster than one with 20 GB on it.