My Mac Mini was to slow with its HDD.
I installed a SSD and my problem is fixed because iFixit.
easy! with my iPad running youtube how to do it, step by step..
Plug and play, formatted my SSD and it automatically asks for Fusion Drive. click OK!!
SSD is a big step up in speed. to make it all extra easy use the Dual Drive kit from iFixit. It wont go wrong.
Create a backup. cleanup your HDD, and you can Fixit.