I think they'd do well if they could speed up the ticket machine accepting contactless: it only takes maybe 5 seconds to go through and then beeps, but 5 seconds per passenger per stop adds up. They could also add a light to show that it's been accepted, sometimes it's hard to hear the machine go beep and then you have to wait for the driver to wave you on.
The issue lies with the passenger. The actual tap goes through in a second, and the system can process multiple separate taps even if it doesn't beep immediately.
Passengers who don't have their fare device ready and spend 5/10 seconds struggling to unlock that device or the classic run for bus then bag rummage is what causes hold-ups.
I would also say that u-22 concession cards can take a considerable amount of time to read on the Wayfarer due to them being a multi-use card, for school libraries and canteens etc.,
I feel the Wayfarer system Lothian are using has probably been pushed to its limit, as its not the most stable system as used from my point of view as a Driver.
*Wayfarer - Ticket machine used by Lothian.