And the trains are not fast because journey times to Aylesbury are much the same as slows.
Actually, in some case the limited stop services (I won't refer to them by the laughable title "fasts") are significantly
slower than the all stations services.
For example, both the 07:39 AVP/07:44 AYS and the 08:09 AVP/08:14 AYS M-F up services (which run non-stop to Marylebone from Amersham), since the start of this current timetable take a whopping 62 minutes to complete the journey from Aylesbury (67 from AVP). That is compared to 52 minutes in previous timetables, albeit with an Amersham stop that was added last year to the 08:14. The all stations, off-peak service manages it in 56 minutes. Even the venerable old Class 115 units, on all stations stoppers, didn't breach the 1hr journey time in the late 80s/early 90s.
Likewise, the 19:04 down takes 64 minutes, a whole 13 of which are timetabled between Stoke Mandeville and Aylesbury. Reason - the train stands at the Stoke Road signal awaiting the passing of the Freightliner Cricklewood bins empties thorugh platform 2 (which have stood in Aylesbury North loop for over an hour, awaiting a path on the Risborough branch), whilst platform 3 is blocked by an up service. Once upon a time, the 19:09 (as it then was) was a stopping service, the 19:23 being the "fast" (which subsequently slipped forward to 19:20, then 19:15, now 19:04). If the paths are there on the Met, it would make sense to recast these two services, to ensure that the "fast" doesn't conflict on arrival at Aylesbury with the Freightliner empties.
There is intense dissatisfaction amongst Aylesbury commuters via the Met line at present, which has been raised at many Meet/Tweet The Managers sessions. There are promises of changes in December 2012 which, hopefully, will address some of the current problems.