If the larger file write had occurred at the same transfer speed as the smaller file, you would expect that it would take exactly twice as long. It's only when you want to write more than 4GB at a time that you'll notice a slowdown. That means your day-to-day write activities, like saving different versions of files and moving folders around, will usually end up on the SSD even if you already have a lot of data on the Mac Mini. The reason is that even after you've loaded more than 128GB to the Mac Mini, Fusion keeps a permanent 4GB chunk of space available on the SSD for writes. One thing Fusion does exceptionally well is accelerate data writing. Once you surpass that 128GB, the benefits of Fusion vary depending on what you're doing. 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).

Apple mac mini 2012 i7