Until you overwhelm the SSD, everything feels fast. Remember, until you write more than 128GB of data to the Mac Mini, everything you install or write to the disk goes straight to the SSD. The extent to which you notice the benefits of Fusion depends on how much data you have on the Mac Mini, as well as the application you're using.

From that point on, accessing that particular block of data should be faster. After that second ping, as long as you're within the same user session and you've allowed the system to idle for 20 seconds or so, Apple's Fusion software will automatically move the data over to the solid-state drive, replacing a less active block. It makes that call after you've accessed a data block twice. Once the mechanical drive does start receiving data, Apple's Fusion software kicks in to determine which blocks you might be accessing more often (efficiently, Fusion can move individual blocks of data, as opposed to entire files).