Lets say a CR has 100 checks (health check, aura check and so on) a second, when framelock is on, it will NOT go to next frame until the CR has done what it wants to be done. This causes low framerate, because it has to slow down to make sure everything happens as the CR has been coded for. When framelock is off, all the frames goes on as it normally does, and the CR just tries to keep up. If the CR is not coded to work without framelock this will end in very bad performance (low DPS/HPS).
It's a very very basic description, but hope you understand.
So yes, basically framelock tears your computer, so the better the computer, the better the fps @ framelock obviously. It might also depend on operation system, drivers etc. Always keep latest drivers and fuck windows XP/OSX (mac).
If you experience heavy fpsdrops at one boss in particular (Elegon in MSV (the fifth boss)) don't panic - a lot of people, including me, are having the same problem since patch 5.2.