memory management

hey,
my program draws a large amounts of objects, clears the scene using clf,
and then again draws a large amount of objects.
the problem is, that clf doesn't seem to clear the memory of the first round, which causes the program to be very slow when it draws the second round of objects (i'ts the same amount as the first round, and yet it is much slower).
Do you know why this happens/ how it can be fixed?
thank you