I printed thingiverse minion Dave and after 2 failed attempts (1st tore of from bed, 2nd had jitter at about 50%) I slowed the print speed down. The final result was amazing. However the reported print time, 3hrs, was in reality 7hrs. The speed reduction was small and I would not have expected such a dramatic change in time. Did I miss a step somewhere?
No you didn't do anything wrong.
The problem with time estimates in slicers is that they don't know anything about the printer's firmware and physical limitations, so they estimate time based on the (whoefully wrong) assumption that the printer will do only and exactly what told to.
So, if the gcode says "move 100mm at 1m/s" the slicer will assume your printer will take 100ms to perform that action. In reality though, your printer will take some time to accelerate, and it may actually be unable to reach the target speed of 1m/s altoghether, taking a lot longer to perform the full action.
Typically, Cura estimates are OK for makerbots, and Slic3r PE for Prusa printers, as the software is mainly developed towards those machines, but for most other printers the estimates will be sensibly off.
You may also be interested in trying to use octoprint to feed the GCODE to the printer: octoprint monitors the actual elapsed time against the gcode and try to adjust the estimate of the remaining time accordingly, also telling the operator how reliable that estimate is.