Crucial SSDs have built-in garbage collection but it only kicks-in when the drive has been idle for some time. OS X’s many, many background processes mean this is unlikely to occur. As a result the drive slows down over time.
Crucial’s advice is out of date. They advocate holding down Option (Alt on some keyboards) and booting to the disk selection screen, and leave it there for 6-8 hours for garbage collection to occur. However, in my experience Apple has nowadays configured power-saving to kick in here, so that the screen and (likely) the SSD are powered down. The same thing occurs at the login screen that appears during boot if you have FileVault enabled — if you don’t type anything, power-saving will kick-in.
A solution that seems to have worked for me is to enable the firmware password, as I described a few days ago, and then try to boot to the Recovery Console once the password has been set. However, when asked for the firmware password, just type a few letters. Then leave the Mac for 6-8 hours. For some reason Apple hasn’t seen fit to enable power-saving on this particular boot-time screen.
This worked very well for me. Leaving my MacBook Pro overnight has brought about a massive speed boost. It’s just like when I first upgraded to an SSD. Apps start in seconds.