Check out the EntityRegistry.registerModEntity() method you use to register your custom entity:
public static void registerModEntity(Class<? extends Entity> entityClass, String entityName, int id, Object mod, int trackingRange, int updateFrequency, boolean sendsVelocityUpdates)
{
instance().doModEntityRegistration(entityClass, entityName, id, mod, trackingRange, updateFrequency, sendsVelocityUpdates);
}
The updateFrequency argument is what you want. Lower that number and your entity will move smoother.