It’s a matter of (mobile) perspective
You can either :
- Write applications using .NET on a mobile device; or
- Write mobile applications that happens to use .NET.
If you do the first one and the application is even moderately complex don’t expect any kind of useful performance. The almost limitless processing power on today’s desktops and servers is very much absent from the smaller form factor devices. You have to change the way that you think to be successful.