Warm up time for record playing

@fsellet, you would be wise to discount everyone else's hearing. I am just trying to offer a reasonable and very factual reason for the OP's experience.  https://en.wikipedia.org/wiki/Stapedius_muscle#:~:text=The%20stapedius%20is%20the%20smallest,in%20th....if everyone wants to believe something else that is their prerogative. The only other reasons I can think of that would make sense are; a tube phono stage that needs to warm up, but that usually only takes 15 minutes. Vibration of the cartridge's suspension generating heat softening the damper. This would lower the cartridges resonance point and possibly its interaction with the tonearm. It would not likely produce any changes in the audio range. So, we are back to the little stapedius muscle. We can give rvpiano a case of bilateral Bell's palsy and see if the effect goes away. He can pose for an indian death mask at the same time:)

If it was just your hearing changing than digital sources would take just as long but they take 20 minutes compared to nearly an hour for the turntable. 

If you have a tube phono preamp, 30-45 minutes is ideal. 

mijostyn nailed it...any other questions?

Nah! But, I did like the first version of your post better, Slaw. That’s one; thirteen left in the bank. Does thirteen buy me a piece of plastic?  (You just had to go there, eh?)