If your faster computer now requires you, yourself, to move faster, then relative to you, the new computer might run at the old speed, or slower. Hmm.

That explains a lot of what I've been experiencing for quite some time now. Laptop's internal keyboard types fine in grub, but usb keyboards the password is wrong more often than not.

Someone else said to get a usb2.0 hub. This fixes the problem.

USB is a bad standard for a lot of reasons. The hid specification across versions is one of em.

I am pulling this totally out of my ass, and I might be making assumptions to aren't necessarily true or accurate. But, maybe you can run a powered USB 2.0 hub on one of those 3.0 ports. My assumption would be the chain would only be as strong as the weakest link (2.0 hub) that you might be able to get 2.0 performance on those 3.0 ports.

This would at least possibly eliminate or confirm down stepping to 2.0 as a solution

But I have not had this issue and could not tell you if it would work or not.

I don't think the problem is with GRUB.

There are various different ways in which USB keyboards can encode keypresses. I've seen some BIOSes that just cannot deal with some keyboards due to this. The USB keyboard driver that will be in use during GRUB should be the BIOS/UEFI driver. So I would try updating the mainboard firmware/EFI or try a different keyboard maybe? Or disable the GRUB password if that's an option.

Get a cheap USB hub? Other than fixing grub, that is.