It seems to be the IOS, which Apple is no longer going to update for the first generation. I can still start a small game on mine, but it can't handle my main that I now have to revert to my phone in order to play.
The December 5th update affected devices that are stuck with iOS 4.2.1 - 1st and 2nd generation iPods and older iPhones up to 3G (not 3GS) - these are no longer supported.
The first generation iPad can run iOS 5.1.1 so it was not affected by the recent update - should work fine.
I have an iPad first gen and it still plays great. It could be that you may have too many apps running in the background. I'd suggest clearing that out, restart the iPad, then it should run better.
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The first generation iPad can run iOS 5.1.1 so it was not affected by the recent update - should work fine.