Define friction free? It should be silky smooth.
The problem you have described is textbook cable friction. Kink in either or both of the cable or housing. Frayed cable. Something on the rear derailleur loop of housing causing friction.
My guess is it's the rear loop. Most people and even factory set it up too short. There won't be much friction when the cable is in your hands and disconnected from the rear derailleur. Once connected is when there is a problem.
Smooth flowing loops no tight angles.
Post a pic of your rear housing.