After upgrading the storage drive on a Mac to an SSD, on rare occasions it is not automatically set as the Startup Disk. This means the Mac will sit on the grey start screen for an extended period before starting to boot the OS. This is easily corrected by moving through a few menus and correcting this setting:
•Select System Preferences •Click on Startup Disk •Click on the newly installed SSD and select Restart
The Mac will now start booting from the SSD as soon as it's powered on.
If your SSD is slowing over time this is typically due to it not receiving TRIM commands. Performance can be restored with the drive's Garbage Collection feature.