Storing node results in memory?

Why is it that when I change my filter late in the tree it has to rebuild from the beginning? World Machine (similar designed program), for example, give the users the option to have the program store each steps results in memory so that when they're tweaking stuff later in the tree it only rebuilds what it needs to. Is this a possibility? This would be my most wanted feature. Making complicated filters is such a pain otherwise.

I've been making a high quality rock texture that as a whole is incredibly slow (on quad core no less). However, when I split it into multiple filters I feel like the process of rendering each step, saving the result, processing the result into the next step, saving that result, and doing it again for a third time is faster than rendering it all in one filter. Why is this? Maybe I can try a test to see if I'm just imagining it or not.

Well that explains why my split up version works faster as a whole then (being converted to bitmaps for each step). I think a nice intermediate step would be a proxy node that saves a temp file. This would be nice in general so we can choose how much memory we're using rather than one being stored at each node. But maybe that's a lot of work too, though.

Thanks!