Some games take more CPU power than others to run, so if you have speed less than 100% you probably won't have an easy time locking it at 100%. There are a lot of fixes and options in PCSX2 to get things worker a little faster, but those depend on your hardware. That is all trial and error on your part.

Maybe somebody can give you some suggestions, but most of the speed hacks aren't going to work for everybody.