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?
There is no need/possibilty to fix that. This is a common behavior of Fusion Drives.
The Fusion Drive ("CoreStorage" in the pictures below) usually contains a logical volume group, a combination of a SSD and a HDD).
After installing your system and some applications you'll get full SSD-speed copying a test file because all the files reside on the SSD:
Installing more applications and having more "high priority data" the SSD will be full and the only possibility to copy data to is the HDD. The test file will be copied with HDD-write speed:
Over time your TestFile and some other low-priority data will be slowly swapped to the HDD, because you'll have even more high-priority data. The TestFile loses its high-priority status because of low accesses. The TestFile will be copied with even less than HDD-write speed because it has to read/write from/to the HDD at the same time:
Even if the TestFile regains a high-priority status because of frequent accesses a resulting copy will be copied to the HDD with accordingly low HDD-write speed:
Please check this answer to determine where the TestFile may reside and where the TestFileCopy is written to.