IME the "boarding closes" time is usually the time that boarding opens in reality (at least on budget airlines in Europe).

And for the regular travelers among us it’s 40min before departure - using the fast lane and knowing that boarding won’t close earlier than 15min before take off

Depends how much you trust the trip to the airport. I used to be a 7 minute taxi/30 minute walk away, so I'd leave home at t-45, if the taxi didn't turn up I'd still have time, if it did turn up I had time for a coffee in the lounge.

Now I live further away it's much more variable. If I'm travelling from Heathrow T5 you have to be into security at t-35, so off the train at t-40, so out of Paddington at t-62, add a bit of leeway means leaving Paddington at t-80 to t-90 depending when the trains are due.

If it's a long haul where there only one flight a day I may well push it out to plan to arrive at an airport as early as t-75, which while excessive, is just in case there are problems like punctures or whatever.

Boarding closes 10 minutes before. If you only have your carry-on luggage, and if you're lucky with lines at metal detectors, it takes ~20-30 minutes from the time you enter the airport to get to the plane (YMMV for some of the larger and busier airports).