I think the usb 2 is the issue.
I use a delock thunderbolt enclosure with a ssd inside on my 2011 imac, and it really speeds up the computer vs the internal hdd.
I would either look at a thunderbolt enclosure/ssd drive, or an internal ssd install (bring mindful of the runaway cooling fan issue if replacing the stock hdd.)
[doublepost=1508383177][/doublepost]FYI, there are some devices that will add usb 3 via thunderbolt, but as far as I know they will still not allow the usb 3 drive to be a boot drive, on any usb 2 only mac.