This is a tale of upgrading an imac… a 21.5″ late 2015 imac to be precise. We bought it, we used it, it started slowing down.. a lot. various upgrades of OSX and it slowed down even more. It came with 8GB of RAM and is not upgradable at all.
We asked for a quote from a local Mac service company to clone and replace the internal 1TB spinning SATA drive with a smaller 240GB Solid State Drive (SSD). It came back at over $500Can. That was a bit much so we deferred and came up with an alternative. We purchased a Kingston 240GB SSD (about $50Can) and an external USB3 case (about $25Can), installed and ran carbon copy clone on the 1TB to the 240GB and told OSX to boot from the external USB drive going forward.
That worked! The SSD drive, even as an external USB device, still performed noticeably better than the original internal spinning disk (still inside, still spinning, just not used).
After a year or two the Imac started to lock up or crash. The external USB SSD was corrupting. So we took this opportunity to wipe it, run disk utility programs to clean it up, then recloned it, this time with the intention of opening up the imac, removing the 1tb spinning drive and installing the SSD.
The theory was that this would be even faster, being on an internal SATA interface, but also safer, as the external USB method had other issues with cables, motion, coffee cups, etc.
Searching through Youtube, we found a nice little video that said, it’s easy! as long as you have the right tools!
Those tools included a Torx 10 screwdriver, and a couple of plastic wedging tools, one to pry open the display off the case and the other to cut through the double sided adhesive keeping the display attached.
That worked out well and within 20 minutes the new drive was in place and booting up.
Yay! It worked!

Benchmark Times

Internal spinning drive via SATA (seconds)
powerup to logo 10
powerup to login 91
powerup to desktop 101

external Solid state drive via USB3 (seconds)
powerup to logo 8
powerup to login 31
powerup to desktop 34

internal Solid state driver via SATA (seconds)
powerup to logo 5
powerup to login 17
powerup to desktop 22