I have noticed the same thing, my iPad1 is slow after iOS5, and apps crash all the time. I have been reading about the "solutions", but turning off notifications or the iCloud service is not a solution, afterall that's what iOS5 is all about.
I am quite annoyed that the iPad1, not an old device by any means has been left in the dust by Apple already. Not only has iOS5 slowed everything down, there is no multi gestures, no airplay mirroring, the features that the iPad2 has. Look I do understand certain features need more computing power etc., but I repeat, the iPad1 is not old.
I am not about to buy a new iPad every year Apple, so by not being fair, and providing the same features to all devices is bad marketing! Yours is not the only tablet on the market these days. You have a very good thing going, don't ruin it!
I LOVE my iPad and can't stop singing it's praises but I WILL NOT buy a new iPad everyear to keep up, that's just ridiculous and is a bad consumer mentality.