Q: Slow startup after SSD installation due to grey screen

Have you reviewed this article?

How to choose a startup disk on your Mac - Apple Support

I used the same Samsung Evo 500gb. SSD in my MBP mid-2012 13-inch.

I had a grey screen and slow startup also however only on the first boot up.

After that it boots up fast. I used "Carbon Copy Cloner app" to make a clone of my Hard-drive.

Are you saying that the MBP will boot with the new SSD, but slowly, and that in System Preferences>Startup Disk there is no drive displayed?

Ciao.

Did you replace the SATA interconnect cable as well.

Maybe if you reinstall OSX and only make a backup of the homefolder of the old harddisk.

that way the system errors don't get over on the new SSD.

Was this just the first startup or repeated every time after the first start up. If you did not hold the keys to select a start up disk it was just looking for the old hard drive. Nothing to worry about if it is not doing that anymore.

Delete the System Preferences plist.  That may correct the Startup Disk problem:

https://discussions.apple.com/docs/DOC-4301

If not, you may have to reinstall the OSX.

Ciao.

What happens when you power on the MBP and hold down the "option" key then select to manually boot from the hard-drive? Is it still a slow boot up?

Same here.

Clear the PRAM/NVRAM and SMC, if there's a drive ID mismatch, it'll give you a grey screen with nothing on it for 15-30 seconds.

Whenever you change out the boot drive on a Mac, it will try and boot from the last-known drive ID (it may cache the ID of the drive in NVRAM)

How to Reset NVRAM on your Mac - Apple Support

Reset the System Management Controller (SMC) on your Mac - Apple Support