Google's attitude towards Windows Phone shows nicely how much they strive for leveling the playing field. Or we could look at Google Maps on mobile IE -- in WP 8, it is horrible, in GDR1 it works very well, all that's changed is the user agent.
When it comes to mobile IE performing poorly, I don't think you need to look further than the software itself.
See "Misspelling "Windows Phone" Makes Google Maps Work".