I just completed the install of a 256GB Crucial M4 into my mid-2011 21.5" iMac last night and the drive is very slow.

The boot takes ages, the system is almost unresponsive when it does get to the login screen, and it is wholly unusable. I opened the iMac back up and made sure the SATA cable was plugged in securely at both ends and checked for crimps in the cable as well. I re-cloned my hard drive just to be sure that wasn't the problem and a 100GB clone took over 5 hours. When I cloned it the first time using a USB-SATA cable, it took less than one hour (and booting off the drive using that same adapter cable led to speeds that were still faster than my HDD, leading me to think that the drive is not bad).

Any suggestions?