The first Rails framework was fast. then it become slow, big and the idea of convetion over configuration becomes more poisoned.
Currently to start with Rails you have to learn a lot of stuff which are added just now and is not sure how long they will stay.
Also Rails is not targeting the developers to be fast but is targeting to be the perfect framework.
The price for that is that almost a year and a half there is no new version while the they refactor it.
No goodies like scafolding, only enterprise stuff around.
That is my personal opinion on this framework.
As I like to say – you cant leave your followers without food for such long time.