Mybb2.0 and yii

[ZiNgA BuRgA]

If you assume that developers are equally skillful, spend an equal amount of time developing it etc, a hand-tuned framework would be more efficient than a generic one, but that's not necessarily bad for users.
It could mean faster releases and a more stable platform, for example.

Besides, I wouldn't stress about resource usage at that level.  If it's that important to you, you probably shouldn't be using PHP in the first place.