Fix a DIY Fusion Drive

A few weeks ago, I took out my early 2011 13-inch MBP's optical drive, put my 500GB HDD in the optical drive area, put a 120GB SSD where the HDD used to live, and made a DIY Fusion Drive according to the instructions at Macworld's website. I got amazing results at the time (a formerly-1:21 boot time became 0:18!), but a few weeks later, I tend to get rather un-Fusion Drive-like speeds. I've seen Fusion Drives duplicate a 4GB file in 21 seconds (source), but a test run I did just now took 1:31. What can I do to restore my Fusion Drive to its youthful glory once again?

What make SSD?

Samsungs may need a firmware update & performance restoration to prevent slowdown [scary, done in DOS, using very inadequate instructions from Samsung, I've not yet braved it]...

...or you could try TRIM Enabler which may go part way towards speeding it up again.