I just completed a successful reassembly of a Suntour 7 speed freewheel, and I had an issue similar to yours. In the rebuild I used Park Tool PPL-1 grease. I used a bead on top and bottom to place the ball bearings and I slathered it a little too liberally over the pawls. Once put back together, it felt smooth but really sluggish. I never even got the freewheel on the hub. I WD40'd it and removed all the grease and re-lubed it with Triflow lubricant. Much thinner and it is much less sluggish. It's back on the bike and it feels great. I anticipate I will probably have to give the freewheel attention at more regular intervals, but that doesn't bother me as much as the extra friction did.