CafeTran

 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!

Can't you create smaller VMs with one mayor app per VM? Besides that: a VM is actually a folder with many files, instead of one big file. And: why should your external disk get disconnected? What would be the loss? Not the VM itself, since you have cloned it (didn't you). Neither the ongoing project, since you have it mirrored in Dropbox (didn't you). And yes, always buy the heaviest iMac. With a big SSD. You buy it for three years and then sell it. For a very good price.

>> a VM is actually a folder with many files, instead of one big file

This is good news. OS X may pick up some WIN apps and move the to SSD. Thanks!

>> why should your external disk get disconnected?

Aha, my wife sometimes sneaks into my work room to vaccum-clean while I'm away for a break!