I have heard Jamil and others say that it takes a couple of batches for reused yeast to really hit its pace and after three or so, it really rocks the house. The only thing I would be worried about using the whole yeast cake time after time would be dying yeast cells and trub building up. I have read about rinsing your yeast to harvest the healthy cells and leave behind the dead ones, but I have not done it. My brother does it almost every batch and says it is really easy. There was a Brew Strong podcast about it a while ago, you can find it here:
http://thebrewingnetwork.com/shows/543The fastest I have ever had a fermentation finish was probably 3 days.