It a few commands to split the Fusion drive into two separate internal drives you can manage independently.
Either way, "Slow as molasses" my arse.
Why does the "manage independently" concept still exist? I've used Fusion, with absolutely no need for human intervention, since early 2014. I also have an all-Flash iMac. In day-to-day use, I can't tell the two apart. Both boot about as quickly, neither bogs down waiting for large resources to open. To borrow a well-worn phrase, it just works.
Further, because Fusion moves blocks, not entire files, it can accomplish something I can't do manually - move only those
parts of a file that are actually called by the CPU. That's why entry-level Fusion drives manage to get by with just 32 GB of Flash.
Modern apps and OSes are highly modular. How can I possibly know which modules out of the the thousands of items in my 13.89 GB (macOS) Library and 10.95 GB System folders are needed at this moment? There are dependencies upon dependencies. Since I can't know this, "manually manage" means putting the whole bloody OS into expensive Flash storage, and leaving it there. 24 GB of Flash just to be sure some obscure command or plist file is Flash-resident if and when it's ever needed. And this is just as true for apps. If you View Package Contents of the typical major app (like 540 MB of Pages or 2.04 GB of Logic Pro X), you'll find hundreds or thousands of files and folders within. Which of those deserve to be Flash-resident, which sit totally unused?
If I were to truly "manually manage" a limited amount of Flash, I would want to move active data files there, not just apps and OS. Data most certainly benefits from the speed of Flash. So, I'd have to take the time to Move from HDD to Flash, then Move back to HDD when I'm done with the file. Move, not Copy, because otherwise I have two versions of the file and the next time I'd have to sort out whether I should over-write the version in Flash with the (possibly older) version on the HDD, or vice versa. Yet if I forgot Move something back to HDD when done, I'd have to Search to know which drive held that file....
I hate being a file clerk. It's not my job to know what bit and byte I'll need next, or where to put it when I'm done. That's what an operating system is for. I've "hired" my computing devices to do all the fetching and storing, so I can use them for something productive.