1.5 is almost not playable because of lag

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!