I have the same problem. It also takes me a few minutes quicker when i masturbate. Another reason for this could be because alot more pressure is applied by the hand than even the tightest vagina. So overall stimulation is maximised. The hand is also trained exactly how much pressure should be applied where, and at what times. It’s obviously totally impossible for the vagina to do this.
I also agree that climaxing is a mental thing too. I have often found that if I’m really into the session, i climax quicker. Also if there was more foreplay. But I think as we get used to our partners, the mental aspect fades a bit. and then we need 80% or more physical stimulation.
I really feel for you guys, as I have had plenty of sessions just carrying on too long, without a climax. And once I get too tired/hot/out of breath, it makes it even worse.
The mental aspect can also work against you! Because some guys, like myself, feel pressured for taking long. And once a person feels pressured during intercourse, it will most likely prolong the climax even further. Especially for those quickies. I’m now in a process to not feel pressured, and not force myself to climax. If i don’t climax within 20 mins in a session, then so be it! Will try again next time. Thus slowly the pressure of climaxing will subside. Hopefully this will work for me.
Good luck anyway! Hope this helps a bit!