Notes Client / Designer is 32 bit code even if it's running on an 64 bit O/S.
So pretty much limited to 2 gig max memory.
If you have 8 gig then setting to 1 gig heap should keep things running smoothly.
that's what mine is set to.
Is it always the same application you are working on? Are you remembering to "close" projects / applications you no longer need open?