I have the Optimus S and my wife has the Hero. Angry Birds is a slideshow on the Hero (very choppy) and is mostly smooth on my Optimus. Levels that have more complex physics interactions (lots of stacked blocks, lots of pigs, etc.) will slow down worse than simpler levels. It's a relatively simple game in terms of graphics, but the physics calculations are what cripple the gameplay on less powerful phones.
That being said, the latest version of AB and AB Seasons (just updated yesterday) have an option to turn off background graphics. This allows for an incremental increase in processing power available for physics calculations. It has made a noticeable difference on both my Optimus (completely smooth all the time) and my wife's Hero, but it's still very difficult to enjoy it on the Hero.
I would upgrade my wife's Hero, but now with Sprint's $10/mo upgrade tax, it just doesn't seem worth it... plus she doesn't really like AB anyway.
The Optimus phones have more processing power. There is a Lite version available in the market, has anyone tried that version?
The "Lite" version is an early beta. It would likely offer an even worse experience full of bugs and fewer performance enhancements. But if you want to try it, it would only take a couple seconds to download and install...