If my memory is correct we had to make some questionable changes to cycles to make shadow blur work in P11. We're investigating implementing those changes in P12. But, if you compare render times between P11 and P12 for the sample scene you'll find the P11 render takes roughly 6 times longer. If it's truly the Shadow blur hack that is causing this performance hit we may have to look for other solutions. On my system the P11 tender is 12.2 seconds where the P12 render is 2.3 seconds. Ouch!