Yes, that is just what I was talking about. Brush work seems to be LR5's Achilles heel, you can sit a wait for it to recover quite a long time. Any suggestions for this particular problem that several folks have now chimed in about.
I had the problem with LR4 too. I only wish there was some way - maybe an 8 core 3.5GHz machine? Priced one out yesterday - I'll be sticking with my quad core for a while yet. I don't think it will be solved soon - the brush is very processor hungry.
When re-opening a previously worked on RAW with a considerable amount of brush, gradient, or spot removal edits, the file will still be sluggish for LR to work with.
A workaround is to export then import the file as a TIFF. The previous edits on the RAW file remain but when working on additional edits, the (TIFF) file is not sluggish (unless the second set of edits become ponderous). The obvious drawback is that the previous edits embedded into the TIFF cannot be undone. At least it's not frustrating to work with.