Fusion can only slow an existing SSD down.
This is a 100% certainty.
Fishrrman, I can tell that you consider yourself (at least on MacRumors) the authority on drive technology. However, sorry to disappoint you but I don't completely agree with you. Apple states that in a Fusion Drive, the two drives joined together in software will perform with the speed of the SSD, so how you come to this conclusion that it will slow the SSD down I do not know. I am using a FD for 2 or more years now and it consistently performs at the maximum SSD speed for my setup.
However a FD with a 2.5" 5400rpm HDD is not necessarily faster than a pure 3.5" 7200rpm HDD.
I see there is some general misunderstanding here. Following on from the above, the rotation speed of the HDD will not theoretically affect the FD as from the user's point-of-view it is performing at the speed of the SSD, not the HDD.
One major disadvantage is the small size of the SSD in the 1 TB fusion drive, and if you have the 21.5-inch screen, the hard drive part is a 5400 rpm 2.5-inch drive.
Finally a perceived widespread misunderstanding is this "small size of the SSD in the 1TB Fusion Drive". From basic computer principles, when you start the machine up it initialises the hardware then starts the operating system, which provides the user control interface. With macOS, the kernel (essence) of the OS plus kernel extensions (kexts), etc., are loaded into RAM for fast access, a truth that is common for all apps that are subsequently loaded. Basically when operating a computer is working from the data available in the RAM, NOT the physical storage, be it HDD, SSD, FD, FDD, SD chip, USB stick, etc. All data used by the processor (CPU) is taken from RAM, not disc. So, if you have 8GB RAM then that is the working space available to your processor. At any given moment all the data in use by the machine will be in that space.
Getting back to the point, as above quoted by nambuccaheadsau in Post 2,
The current 1TB Fusion has 32GB of PCI-e Blade Drive, also called SSD.
So the speed of a PCI-e SSD is 1 GB/second. This is the speed of the 1TB Fusion Drive in the basic Apple iMacs, according to information provided by Apple. Yes, the drive is only 5400RPM but that fact can be discounted as that matter has been dealt with above. Yes, the SSD is only 32GB but this is 4 times the standard size of the RAM in the machine, so in the case of excessive computer use (lots of memory hungry apps open, etc.) and the need for paging occurs, where 'pages' of data from RAM are written to physical disc to free up RAM space for other data, this transference will be from DDR4 RAM to the FD at 1GB/s. The SSD is 4x the size of the RAM so even if the whole of RAM was paged to the FD, the FD still would have 24GB of free space to work with, for extracting fresh data from the HDD portion of the FD, copying data from FD to RAM, etc. In other words, ample. For the majority of users in the majority of use-cases paging will not occur, as 8GB RAM is sufficient for the majority of common tasks. In that case, new data needed by the computer in the case of opening a new app, saving a file, etc., will all happen at SSD speed, in this case 1GB per second. I hope this is clear enough because I would sincerely love to clear up this misconception that a 32GB-equipped FD is basically useless and should be avoided like the plague. Of course, everyone is entitled to hold their own opinion. I also have one. What I am presenting here though are practical facts that I believe many people do not either consider or understand in total, otherwise they would not come to the conclusions they do.
