Actually, I think there is room for this feature.

the calculated time could be attached to the submitted solution, and when listing the solutions you could sort them by time.

I've explained here why this is impossible. Some other factors are not mentioned there, but it's more than enough to explain why such feature cannot be added.

As I understand correctly, I could write a terribly inneficient solution and still pass the kata?

This depends on the kata requirements.

If you want to solve katas where perfomance matters, look for the tag Performance.

There's no such feature, and there won't be one, as the test setup is provided by CW users.