Factors in freewheel rolling resistance

I bought a new Sunrace 7 speed freewheel recently for a 6-to-7-speed conversion, and I noticed that it spins less easily than the original Suntour (I think a "Winner" model, not sure, don't have it in front of me). The original Suntour will just spin and spin, while the Sunrace feels like it's fighting something. (I have not used the Sunrace while riding, as the build is in progress.) I flushed the Sunrace with WD-40 and added Phil Tenacious Oil, but to no apparent difference.

This sparked the question -- what are the factors in freewheel rolling resistance? Bearing quality? Assembly? I feel like I've eliminated lube viscosity, but maybe I need to work on it more? I dunno -- thoughts?