My speeds were getting terrible (Dec 2012 model, factory 1TB Fusion.) I was seeing < 100MB/sec in both directions. I rebooted to single user mode (command-S during boot) and at the prompt entered: fsck -ffy One of the messages that scrolled by was "trimming unused blocks." After the reboot, my speeds were back to factory fresh! 202MB/sec write, 352MB/sec read Just wanted to share this tip if others feel like their Fusion drive is starting to suck! My guess is that OSX fails to trim the SSD on its own.