Hub sticking - friction
Probably you need to service that hub - or get it serviced by your LBS.  That much drag means that either it's full of dirt, or the grease has completely dried up or water contaminated, or the bearings have collapsed.

None of those are likely to be fatal, but you would be well advised to open that hub up and clean it out, check the bearings, and re-grease it.  Or have your shop do it for you.