1.5 is laggy for me, but 1.47 wasn'nt

My expectation was to log in and play as usual but it lags to much for me to play now

I'm sorry I can't exactly tell how to reproduce but I can tell what happened for me:

1. Was running and playing Minecraft 1.47 and it worked fine (wasn't laggy)
2. Downloaded and updated to Minecraft 1.5 and all of a sudden it's so darn laggy that I can't play!