Depends on the location. A popular stop such as a shopping centre or a high school can already take a very long time for everyone to touch on. If a decent number of those passengers are also topping up, it may well take 5 minutes for everyone to board the bus.
PTV should do at shopping centres what Sunbus does at Sunshine Plaza in Maroochydore. At Sunshine Plaza, there is a dedicated ticket booth at the bus stop with a real person there where you can buy Go Cards, not sure about topping up though, but that would be a good idea.
Mind you the buses are parked differently here, which would make it a bit more difficult to implement. In Chadstone or Southland, the buses are parked on the roadside in long lines along several different roads, whereas in Sunshine Plaza they are angle parked next to each other, which makes them very close together so the ticket booth is near all of them.