My mac performance was at its least with OSX Maverics
Initially, i was 90% close of replacing the SSD and failed. Guess what, i didnt have the T8 Torx screwdriver to open the HDD from imac (pss i didnt read the required tool for this step). So i had to put everything back and order the T8. So it took couple of days to get the screw driver (ifixit has the best shipping service i guess - very fast shipping) and finally the process went great.
The LCD (black ribbon connector / Display data cable connector) cable is very very delicate. So be very careful when handling that cable, i broke it as i was not careful but got lucky as it is just the connector handles that were broken but not the connector itself.
Wait, where can we see this cable? I found this at step 7 or replacing hard drive instructions found on ifixit (link below)