Here's a screenshot of my fastest disk speed test on late 2012 imac.
RAM disk? It's probably the only answer because the internal drive is limited to SATA3-6Gb/s (750MB/s theoretical max, but normally tops out at 520MB/s), and your late-2012 iMac can only support Thunderbolt 1, so the max theoretical is 10Gb/s (1.25GB/s).I use that partition for rendering videos in FCP. On an internal drive... Do you want to guess?
For my RAM disk, I hit 3.3GB/s before.You're right! It's a Ram disk. I just thought I would post it here so you can have some ideas. What is your fastest ever disk speed test?