The important question:
What kind of drive is inside?
If it's a platter-based hard drive, you can upgrade to an SSD which will transform the overall performance.
The 2012 has USB3 ports, is that correct?
In that case you have a choice of two ways to do this:
1. Open it up and install internally
or
2. Add an EXTERNAL USB3 SSD
Choice 1 carries with it all the risks involved in prying open an iMac (unless you've had experience with that before).
Choice 2 is fast 'n' easy, but not -quite- as fast (you'll get about 80-85% of the speed that you would have gotten from an internally installed drive -- hardly noticeable).