Once you hit the platter portion of a Fusion Drive, it's a bottom-of-the-barrel 5400 rpm disc, and it's really slow. It will grind to a halt. You may have as little as 24GB SSD in that Fusion Drive. It may be, prior to 2015, Apple put as much as 128GB SSD on a 1TB Fusion Drive. Apple has mucked around with SSD space on Fusion Drives to save money, and around this time reduced it to 24GB SSD. That is really low; a $10 usb flash drive comes with 128GB, or 5.3 times the amount of fast storage you get with Fusion. I don't know the exact proportion of SSD/5400rpm in this particular machine, so you should research that.
We tried using an extra Mac with a Fusion Drive as a working backup drive. It sucked. Everything ground to a halt when we hit the platter, and we hit it often, resulting in spinning wheels and a paused machine. Felt like the machine was ten years older than its release date.
Main point is, spinning platters for your main working drive is a mistake. Platters should be used only for true non-working backup media, because they are cheap and because it will be needed only for data recovery and storage. Your main disk, and any working backup media, should be 100% SSD. The speed difference is monumental, especially if you are working with a lot of data, files, running databases, compiling, video editing, etc.