20 min is a ridiculously short flight time for a properly conducted LPC.
Maybe, maybe not. I think that in a lot of cases your first impression, the one you had in the first five minutes after starting the engine, turned out to be right. So what's the rest of the 20- or 50-minute LPC going to accomplish then?
For example. Somebody comes to you for an exam. Hands over a wad of paperwork, such as a proper wind-corrected plog complete with calculated take-off and landing distances, annotated weather and NOTAMs, and the SRG\1119 form already filled in as far as possible. Flight plan has been submitted, he knows how much fuel is in the aircraft and whether that's sufficient. Pre-flight has been done and all you need to do as an examiner is get in. Candidate straps in next to you, confidently starts working down the checklist, starts up, taxies out, performs engine run-up, takes off. Good liason with ATC and good situational awareness. Navigation to the practice area, including maintaining course and altitude, plus keeping a lookout, is also spot on. During all this, the candidate is clearly at ease and has spare mental capacity to deal with an occasional question or remark from your side.
How often does it really happen that somebody like that makes such a hash out of the subsequent part of the LPC (stalls, steep turns, PFL, landings) that he/she fails anyway?