TOTO is fixing this, but there is certainly a slowing, if only by a couple of seconds per passenger.
1990s on Oxford Road: say "40 please" while chucking the right change on the tray, driver just hits "issue" and ticket quickly taken, which itself was much quicker on the old ribbon printed tickets because you didn't have to wait for it to fully print and cut off. You could almost do it without breaking stride.
Now: state destination, driver mishears it, state it again, driver selects the right fare stage, you move your phone towards the machine, driver indicates to tap, you tap, slight pause, wait for ticket to fully print, take ticket.
You are probably talking maybe 2-3 seconds difference, but it all adds up on a full decker. It's worth looking at running times on Merseyrail now vs. in the 1970s to see how a load of very small losses of time add up to significant slowing - 11 extra minutes on Ormskirk to Liverpool, and probably more than that on a Southport or Chester. Even adding hustle alarms made enough of a difference to add a couple of minutes, and that's just 5-10 seconds per station stop.
But if you combine dual door and a well designed TOTO system (i.e. not Ticketer's, much as Ticketer has been good in getting card acceptance and TOTO into the mainstream) you will get those gains. It doesn't really need any more than that.
Of course there are other problems - but why should we not address all the problems, starting, naturally, with those within the operator's control?