I think it's a big question of how well-trained & well-motivated your passenger service agents are, whilst you can ultimately get the job very well done if you're managers are like the Gestapo, it's much nicer if you feel valued or work in a nice environment, which my experience with the circus shows we are in the Gestapo bracket, rather than the friendly one. Because PSAs are relatively cheap to train, and there is a string of naive young people who see it as the stepping stone to cabin-crew, we're seen as something of a replacable commodity.
20-minute turnarounds are achievable if you're well trained, even if there's only one person on the gate, I've done this for both 156-seat easyJet A319s & 189-seat Ryanair 738s. The issue though, is that whilst you are efficient, you don't have time to be friendly, and sometimes you are unable to ensure that the boarding procedures such as priority boarding groups can be offered. At my base, on slot one flights (0600-0700) there is usually only one PSA on the boarding for low-cost flights, whilst throughout the rest of the day it depends how many staff are around if it's going to be one or two. Short-staffedness is something that won't go away as the airlines try to squeeze more costs from the industry, and whilst I don't think it compromises safety, it compromises service & the well-being of your workforce.