My late-2014 27″ iMac has had drive issues since shortly after I got it. It has a “fusion” drive, which is a managed combination of SSD and spinning hard disk.
At times, any disk access will hang the system for 10 to 20 seconds or longer. The machine quickly becomes unusable and I eventually restart, which seems to help for a while. Apple replaced it all in 2016 but the problem never really went away. I’d brought it to Apple a second time but they could not find anything wrong with it so I gave up.
I thought I’d try something. I bought a 1TB external SSD and installed a bootable system on it. I’m now running that (USB3.1) drive as my startup disk and things have been significantly faster. This is good.
The only problem so far is that sometimes when I reboot the Mac doesn’t find the external drive right away. The latest occurrence was this morning after a system update. I had to unplug the drive, restart, plug it back in and choose it as the startup disk, then restart again. This is a pain, but I no longer have to reboot often and as long as I can easily get things going again I’m happy with the trade.