So Safari is quite obviously somewhat faster at redrawing pages. This isn't really unexpected.

If you expected that MicroB would run circles around Safari, despite the higher resolution, despite using the Firefox / Gecko engine, despite offering support for Flash, and despite Apple's years of experience on fine-tuning the browser for exactly this kind of device and user-experience, then this was an unfortunate misunderstanding. Speed is not the selling point of MicroB, capability is. The fact that it is as fast and usable as it is is the remarkable thing.

What matters is that the browser is convenient to use and gets you as close as possible to the desktop browser experience. Both Safari and MicroB have some things going for them in this regard. But this kind of analysis and direct comparison of rendering speed is ultimately pedantry, and if you truly think that this is what matters most to the average user, then I believe that you will be in for a surprise.