It was never a slouch. 2010 iMac, 2.9 quad i7, 8GB RAM, 1TB, 1GB video RAM. I maxed it out when I bought it back then. Very much a processing beast (for me at least) but the HDD was clearly the bottleneck. I was getting read times 70-80MB/sec. After buying a Macbook air with much less horsepower on tap, but with the SSD, the iMac felt slow in comparison. Apps would take a little while to load, but then were responsive. iMovie exports on the iMac were always WAY WAY faster than the Air, but day to day use the Air felt faster.
Now it smokes my Air... logging on, after it boots up to the list of users, is practically instant.
Most dramatic upgrade I've done on a computer (and I've upgraded CPUs, memory, drives in the past- not on a mac).
I will warn to potential upgraders, OWC breezes through the fact that getting to the SATA ports on the back of the board is tricky/difficult/frustrating, as is re-seating the logic board. I had to unplug some other wires back there to get to the SATA ports- and I really needed the Left hand angled plug, which is on it's way from Amazon. If I had to do it again, I would probably connect the SSD before I put it in as well. It's a tight spot back there.