Performance increase from Fusion Drive I have a 2011 27" iMac i7 3.4gHz with 24 GB RAM. The startup disk is a 256 GB SSD and then I have another 1TB HDD installed as well. I use it for both design (Adobe Suite, Sketch, etc) and front-end dev (node, mongo, etc) as well as a little Wordpress dev. For a 2011, I can't really complain about its performance - it starts up in under 30 seconds, can handle NUMEROUS browser, terminal and VScode tabs and run Illustrator and Photoshop pretty flawlessly. If I have a server or 2 running in the background, some of that starts to suffer.

Based on all that, I'm inclined to think "if it's not broken, don't fix it", BUT I just read about the thing people with 1 SSD and 1 HDD have been doing for quite a while now, which is combining the 2 for a Fusion Disk and have to say I'm a little more than intrigued.

Doing this would require me to reorganize all my applications, project folders, scripts, etc so it wouldn't be a small undertaking, but if it will give the boost in performance many have reported, it will probably be worth it.

Given my system specs and the type of work I'm doing on it, am I likely to see a noticeable change?
I would say: No. 

My experience of using a Fusion Drive in a 2012 Mini for several years is that the shared performance benefit lessens as the drive fills up. 

Secondly, and perhaps most importantly: a Fusion Drive has two points of failure, and a nine-year-old hard drive is increasingly likely to fail, at which point you will have only your backup. ;-)

Your best course of action is to use the 256Gb SSD as your System disk and farm out user data 'overspill' to a faster external drive.