Alright, thanks for the advice. Had it set up, but it seems like the crankbolt is coming loose. Very weird, and very annoying as it creates play and makes it nearly impossible to ride with.
To stop the crankbolt coming loose,
1. back off the lock ring, then
2. tighten the crank bolt. (Use a torque wrench - it has to be very tight, but you need to know exactly!)
3. last, adjust the lock ring. If the crank still comes loose then contact Rotor.
New bearings are often tight, but if the lock ring is adjusted before the crank bolt is tightened (3 before 2), for sure there is axial overload on the bearings. This will add a lot of bearing friction, which would give the slow crank results you have.
P.S. Make sure the steel crank remover ring (a Shimano cassette tool fits in it) is positioned correctly in the crank. It should be installed in the crank before the bolt, and the planar face must be in the crank, with the open cup side surrounding the bolt head. Use lots of grease between every joint.