I'd suggest turning down your max plot readers # to test. Each is a thread and unless you have the bandwidth and CPU to actually scan 14 devices at once AND verify with three threads you're going to be going nowhere fast.
I have a mix of SATA and USB drives on my machines and I get the best overall processing speed (read + verify) with a plot reader number of 2-3. If you go too high you'll get seemingly faster reads but verification will lag and add overall time. In your case I think it's WAY too high.
Also Increase your buffer size a little if you have the ram 1024MB for 60TB would be good, 128MB is choking your read times I think, as the CPU maxes to fill the tiny buffer, then goes to verify, and then has to wait to fill the buffer again. It's completely overloading your CPU as my first guess, in a terribly inefficient way.
Also as Tinycoins said, get the newest version (1.6.3), it has a scan bar that differentiates between read data and verified data that makes it easier to visualize and optimize the process.