Does it matter how far backwards you can spin your drivetrain?
I don’t think it’s a good indicator. Plenty of viscous lube and lightweight (-&gt; low inertia) components can easily mean that a high end bike spins worse than a 20 year old bike with dry and rusty bearings.
The most important thing is friction under load, which is hard to measure. But you should make sure that bearings spin with little force (to make sure they don’t have excessive preload), that they don’t have any play and that they turn smoothly through the full 360°.
To check if the hub’s free wheel mechanism has excessive friction, spin the rear wheel. If you have quick release axles, make sure they are not excessively tight. To check if it’s the bottom bracket bearings (more likely) take off the chain and spin the cranks. If it’s neither of them it has to be the rear derailleur.