The face to face lasts c.20 mins with a job based competence test beforehand, that shouldn't take anymore than 10/15 mins. Our kids when they've been for jobs have all had a test and interview, that lasts little more than an hour.
You cannot compare OASC etc to the average civilian company process, given that the MOD employs people to do just that and needs to justify the job/role, ergo 2 or more day process. I did OASC 35 years ago and spent more time in the "waiting room' overall, luckily I took a book to read. I did feel it could have been done in one day. Based on the guide for commissioning and basic interviews for Commission and SNCO in the ATC, they are formulaic processes and time bound.
In the real world interviews are carried out by people who will have to go back to their desk and catch up, after spending the time doing interviews. Also people coming for interview have busy lives and other things to do. I don't doubt there are jobs out there where the selection process is protracted for their own reasons, which people can reference.
As for first impressions, counts for a lot and is like it or not a well held practice. Things like appearance, how people come into the interview room, conduct themselves, even time of arrival, go into this.
As for people spending ages explaining themselves in an interview, this is where the personal spec / supporting evidence part of an application comes into its own. You should be able to articulate yourself in writing and not just verbally. This information provides evidence of people's experience/knowledge, vital for sifting and questions guided by it. Even though I have no intention of moving from here unless I get made redundant, I have put together the parts for a job application. I've given it to the personnel manager to get their take and they said slightly too long, but should get me to the interview stage.