A few data points to throw into the mix, derived from a month-long UK/Europe trip from which I returned a few days ago.
Frankfurt: Arrived at T1 from SIN early eve in final week of May. Did my first EES registration at a kiosk (ppt scan, photo, fingerprints) then queued for about 10 mins to see an agent at a booth, ppt stamped. Dep to LHR mid-morning next day which was a few days after opening of new T3: e-gates operating for EU only so all non-EU ppt holders were directed into a slow-moving queue of 100s of people: it took 1.25 hrs for me to reach the front and be processed by an agent at a booth, with ppt stamped again.
Caen/Ouistreham (ferry port): Arrived on ferry from Portsmouth first week of June, foot pax processed at two staffed booths but took barely 10 mins with photo, fingerprints done by the French border agent. Agent could see in the system that I’d used a UK ppt at FRA but was now entering France on my AU ppt (which he did not stamp).
Milan: Flew CDG-MXP so no EES (intra-Schengen) in mid-June. Departed MXP-SIN earlier this week, afternoon flight. Airport very busy so anticipated significant EES delay but no wait for a kiosk and multiple e-gates were in operation for holders of EU and third country e-passports, so the queue was moving quickly. Immediately after going through the e-gate there was a staffed booth where the agent stamped my AU ppt without really looking at it.
As others have noted, there are inconsistencies and inefficiencies in how the system has been implemented and (not surprisingly) queues can be long or short depending on time of day. FRA seems consistently poor, though. I’m puzzled by outbound delays being longer than inbound, given that all non-EU outbound pax will have done EES registration on first arrival into an EU/EEA country. And all the blurb about EES continues to say it’s replacing stamps in passports despite the stamps actually continuing at some ports.