Not the best forum for this post; try Software or OS X/9.
In any case, under OS X the user has no control over the memory allocation (or if it does, I don't know about it). If InDesign is laggy, it's for another reason.
BTW, in general, when asking for help, more information is better than less. Have you been using the program for some time, and the problems only appeared recently? What computer are you running it on and what are its specs? What other programs are you running concurrently? Have you done any system maintenance recently, or are you nearly out of hard disk space, or have you had electrical disturbances... etc. There are many possibilities, all of which have their own solutions.