Namely, aging effect was not known or fixed in semiconductor processing. So, as flash ages, it disfunctions. But if you warm it up, it will anneal and recover. You have to use flash often, or it will slow down.
It can slow down so much, that beep code can be triggered. But just by warming the motherboard and use it more often, it will recover and speed up to boot quicker. Especially, you will notice the boot logo goes away faster.
Good luck. Don't fix it if it ain't broke. Just like old electronics equipment, you need a time delay to warm up before you boot. Or warm up with error, and reboot later?
I know Dr. Groove personally when his then boss Dr. Robert Noyce introduced us.