Heya!
Perhaps it's just installed on a fast HDD? Or like Barty1884 said.
If you can get into your BIOS and go to the BOOT options and check in which order they are ( or provide a picture here ) then you can check.
If it has a list of multiple devices, it'll use the first one which is availlable, if not it'll try to use the next one, and so on...
I hope this helps out.
Cheers!