Ian from AppMobi here - You are incorrect on your assumptions. We do not get "speed performance from scaling down the resolution".
We get speed performance from layering off the canvas calls and using an OpenGL layer. We attempted to explain this to you in the emails about possible integration.
For everyone else, the way DirectCanvas works is we have two "webviews".
1) Normal AppMobi Webview (phonegap would be the same)
We have a bridge to communicate between the two, but the DirectCanvas webview isn't a normal webview. We have a fake DOM in there, but support is minimal. We implement some of the html5 canvas calls (not all).
This is a short version of it. Here's a slide our CTO did at NewGameConf