We don't know where the delay is happening and we know nothing about the specs of this computer, version of Windows or even if a notebook or PC. But it could very well be a driver issue.
You say a long time, but don't tell us how long? 10 minutes? 2 minutes? How long did it used to take. When did the problem start?
Can you boot into Safe Mode? How long does that take?
If you power down the computer, unplug it from the wall, open the side panel (assuming a PC), touch bare metal of the case interior then pull the power cable from the boot drive, then connect the power cable for the computer back into the wall and boot, how long does it take to get a "boot drive not found error"?