For those experiencing speedups with portrait/landscape, I'm betting it's because of the required reset. Far as I know, all those functions just use the standard GAPI or similar. This driver, as nice as it is, is only intended to work with things running D3D mobile. There's a very nice article on this on the MSDN website explaining the difference between GAPI, GDI, DirectDraw, and D3DM:
http://msdn.microsoft.com/en-us/library/aa454886.aspx
So any UI speedup is purely placebo.