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).
Meanwhile on my SSD, I hit 915MB/s and 1035MB/s for read/write respectively (1TB PCIe SSD in my 15" rMBP, strange that write is faster), 750MB/s and 720MB/s for read/write respectively on my 27" iMac (512GB PCIe SSD).
PS I also use RAM disks myself to reduce writes to the SSD in order to prolong its life as much as possible. With 32GB of RAM on my 27", it's no sweat creating a large RAM disk 🙂