Why does each grasshopper recompute take longer than the last?

I have a huge GH1 definition with many elements and clusters within clusters.

At document load it will compute within 20 seconds or so, but recomputing takes longer and longer - quickly surpassing 200 seconds.

Are there any steps short of restarting Rhino to get back to the quicker compute times?

I have found great resources on optimizing your definition, but nothing which addresses this phenomenon of an unchanged definition taking longer with each compute cycle.