You definitely don't need to purge anything... RAM isn't a harddrive, there's no need for your OS to "empty" it after use!
Why doesn't it set this chunks of memory as free? Because it's CPU consuming , and it's useless.
Even if it's not free, it's still available for write.

12Gb is a lot of RAM, the bottleneck is certainly elsewhere.

You didn't say much about your setup.
Do you use your DAW as 32bit or 64? With some extensive samplers (string/piano libraries) it can be a limit.

ps : Memory cleaners are placebos. it does make cool metrics though. But any modern OS do that just fine on its own.

ps2 : I use 10.6.8 on my laptop, it's much less CPU intensive 10.9

 

Can't agreed more , the very best (32 bits only ofc),useb on mac book (very old) and hackintosh ,best OSX to do music