I have 3 devices that run iPhone OS. I've also run every version of iPhone OS ever made. iPhone OS 2.2.1 is the smoothest operating OS for the "iPhone 3G".
It's not false. I've done it all. I also see how my brother's iPhone is slower than heck and he doesn't even jailbreak it. It's always lagging no matter what he's doing and he's on 3.1.2.
However, if you want to use 3.1.2 and you're happy with it, then be my guest because it doesn't affect me at all about which OS you're running. I just know that OS 2.2.1 is fastest and smoothest on my old-generation hardware. I refuse to upgrade to the iPhone 3GS and sign a contract. I'm currently contract-free and LOVING it.