One slow wheel can be caused by an overtightened axle nut, a bearing that is not seated, a damaged bearing, wheel contact or a spacer problem.
Swap and compare
Spin the wheel by hand, then loosen the axle slightly and check again. If the problem follows the bearing or wheel, you have narrowed down the cause.
Fix the cause
Correct the hardware order, reseat the bearing or replace the damaged part. Do not compensate for a rough bearing by leaving the axle loose.