Why is MVVM taking longer? What about it is? Is it MVVM, or is it due to use of WPF? Are you measuring 'taking longer' as initial implementation, or are you considering longer-term maintainability? What does your code look like?
Without knowing the answers to some of those questions, it's hard to get a good feel for how to help. In my experience, well-written MVVM code should be faster to develop than 'put all the code in the UI' style code, not to mention the testability, maintainability, and flexibility advantages.
(answering questions in comments here, since it's a long answer)
I've done my wpf work just using wpf. I don't bother with 'base' view classes unless I need them for some weird reason. A VM should be explicitly for a single view (dialog, etc.) of the data, so a base would not make sense. Having an interface for it usually doesn't gain much either, as typically what you want to do is bind the view to the viewmodel, and have the model abstracted for testability.
In short - don't overthink it Your viewmodel just presents the data the view wants as properties, and accepts commands from the view either using commands or plain methods. That's it.