Can you uninstall updates on IPhones?

I just got an IPhone 4 and some people were saying that the update iOS 6.1.2 will slow my IPhone down and others say it won't. So is there some kind of way that if I download it(to see for myself) to uninstall it if it does so down my IPhone

I think once you put it on, you can't take it off. I've heard good and bad things about iOS 6 as well, so do some more research for yourself (not from apple sources). But wait before upgrading, because if it sucks, you will be stuck with it.