Garmin Drive 51 slow to boot

One cause of a slow boot is corrupted maps, you could try removing any third party maps and / or re-installing any Garmin maps

 

You could try a factory reset with :- Main Screen >> Volume >> [touch and hold top right corner of screen for 5 seconds] >> Clear All User Data >> Yes

Note that this with remove all stored Tracks, Routes Waypoints (Favourites) etc. so you need to copy anything you want to keep to a computer before the reset, also after reset the first power up will be slow but following power ups should be quicker