I have noticed on the few times I have let the temp crash too cold that ramping the temp up (say, from 50 to 68-70 over 3-4 days) quickly accelerates fermentation time with no off flavors (assuming that yeast can work that cold.) Also, pitching a second batch of beer on a equal size beer 24 hours into its growth cycle really speeds up fermentation with no off flavors. So, say you brew 5 gallons of beer day one, aerate and pitch an adequate yeast pitch for that size beer, then put 5 more gallons on top of that 12-24 hours later you will drastically speed up fermentation time. Just be sure to aerate each batch well.