As low-cost airlines shun the use of airbridges, the trend seems to be that it's actually faster to get pax across the ramp and up two flights of steps where they can spread themselves about where they like. The problem is that most people only see one flight of steps, or do not want to walk to another set especially if it's cold & wet, and furthermore what I've noticed with bmibaby, where we have assigned seating, at EMA is that our CSAs no longer direct pax to a particular flight of steps. Go, who used the same model as we do, used to direct pax sitting aft of the emergency exit rows to board through the rear doors, and those forward to go through the front door. I agree with FougaMagister when he says that pax with special needs don't seem to be accounted for with these fast turnarounds, whether they're slow walks or actually need an ambulift. Not having airbridges presumably requires more staff anyway, as pax have to be directed across the ramp & assisted with steps, so does time over number of staff save money?
I'd say 30-minutes is plenty of time to turnaround a Boeing 737, after all even post 9/11 Southwest (before they had 15-mins turns in some stations including LAX) in the USA manages to do it in 20mins. 30-minutes not only gives the gate staff enough time to do their jobs in a professional & secure manner, but also is much more comfortable for both them & pax especially those with limited mobility or other special requirements. 20/25-minutes just rushes everyone, and I wonder what added expenses would come with just adding 5-minutes.