Slow boot on Ubuntu 16.04 with Samsung 850 Pro

If GRUB isn't on the SSD ( which it should if properly configured during the installation ), then just having the SSD there will take GRUB time to recognise it, but it will not in any way decrease boot time, actually exactly the opposite. What you should do is make the computer boot from the SSD, by changing your BIOS boot order. Note that you also have to reinstall GRUB on the SSD. Although in some cases like my laptop you can't select SSD as the boot device from the BIOS, then you are stuck with going to the HDD, loading RAM, then going to the SSD.

Also this is what I think happens, but I don't fully know your laptop or desktop configuration, so this is all I can help with.

Hope this helps. :)