Hi,

It should be from the BIOS, but if you change your BOOT mode, you will likely have to reinstall Windows. You should also check if the boot order is properly set and if there are not too many startup items slowing down the power on process.

The is no apparent setting in the BIOS for AHCI.

Then the boot setting is automatically set for that. How long is it taking to get into Windows?

Maybe post your R/W speeds and IOPS.

Samsung Magician often has trouble detecting AHCI. I would not automatically assume something is broken.

Also, if it's 2.5inch or M.2 form-factor

SATA or PCIe interface and finally

AHCI or NVMe protocol.

I went back to the original windows 8 and then upgraded to 10 again and everything is now working just fine.