That's not a bad point. After all, two minutes had to be added to Ormskirk-Liverpool Merseyrail running times in the 1990s (which actually necessitated adding an extra unit and crew, if I recall correctly) when hustle alarms were added, thus adding just 10 seconds to the dispatch time. (Move on a few years and that unit is needed for Maghull North anyway, of course)
FWIW, while the timetable allows for it so it doesn't cause actual delay, the running on Bedford-Bletchley is rather sluggish, and I've observed on many occasions that delayed door release while waiting for a transaction to be completed is a common cause. I reckon you'd probably get a couple of minutes off that as well - no great issue as it's self-contained at both ends, but might give a better impression to passengers. Added to the likely gain of a couple of minutes from the higher acceleration of Class 230s as compared to 153s, that's starting to get noticeable. 46 minutes down to 41 or even 40 is potentially something to shout about, even if it's poor compared with driving.
The other thing to bear in mind is that a wheelchair passenger may well be located at the opposite end of a train from the guards cab. Having a guard right next to the wheelchair on approach to a station ( without having to worry about opening doors ) is a time saving as well. I ve lost count of the number of times I have had to make that journey.
)