Touch'n'oops,
It's nowhere near as significant as a windmilling propeller, BUT, the windmilling engine is extracting energy from the airstream (Something is making it turn, and energy doesn't come free). The Performance Manuals for various heavies give accountability for the worst case, this being windmilling failed engine/s. (Usually found in the Performance Manuals in association with the offset rudder drag coefficient, which of course also goes along with a failed engine).
I can only believe the books, I've shut down many during test flying, but so far have been unable to make one stop rotating altogether - don't want to thanks!
Regards,
Old Smokey