Much of the slack at Birmingham is due to DafT's dismantling of the original Cross Country journey patterns so that the timings on the individual legs no longer match up. Originally the Bristols went to Newcastle, the Readings to Manchester & the Plymouths and Bournemouths alternately to Glasgow and Edinburgh. There was also a Brum - Manchester working. Long layovers in New Street were rare. The current arrangement cannot help the capacity problems at this location.
I think similar layovers at New St have been in the timetable for years, (I was a regular Newcastle - Plymouth traveller in 1972!) and the recent changes to the
XC▸ network don't appear to have made it any worse, they've just moved the dwell time to and from different routes.
Previous timetables show the Bournemouth to North East & Scotland always had the long delay (about 18 mins) but now they all go to Man Picc it is much reduced.
I think it's still mainly down to national pathing problems, it is just too difficult to shuffle every service in the country to make the stops shorter. Anyway, with all the extra changing at New St, presumably 3 min stops would not be long enough?
Paul