I recently bought an iMac 5K with 1TB FusionDrive (Congratulations!)
But I immediately found that Win10 on Parallels works incredibly slow ...
Well, 1TB FusionDrive comes with 24GB of SSD, but my Parallels virtual machine exceeds it, of course (about 40GB).
Because it is one whole file anyway, there might be little chance that frequently used WIN applications will someday be moved to SSD.
Oh, I should have bought at least 2TB FusionDrive, which comes with 128GB of SSD, or simply 128GB of SSD!
An external SSD can be an option. But I'm afraid that inadvertent disconnection during use might corrupt the file.
If you have a plan to buy a Mac and run Parallels, be careful!