My single tumbler for a 16" turns real easy, like butter when I was done with it. Not sure if you used a felt kit from ebay or not, but some felts seem to fall into a bit of a gray area, like it was real close to use one a little thicker or thinner. Felts too thick could add drag.
One suggestion, if you have not mounted it, roll the case over so gear end is facing you. Roll each shaft, and each of the non-fixxed gears back and forth to feel the gear lash. That'll tell you if gear lash is too tight(probably not if you using same gears)but you may find that one gear, or one shaft is harder to rock back and forth, while the rest roll easy.
Also can check end thrust on shafts. I would not leave zero clearance. Maybe .002-.004" for oil clearance. If the bushings for shafts are knocked in too tight could be dragging.
The last thing I can think is the tumbler gear and shaft. As is, getting them used I would expect a repair needed as the bore in tumbler arm wears out. If the gear and shaft can move up and down significantly the gear will bind on occasion, depending on which notch you're in.