Do dry wheel bearings increase drag?
If your wheel can spin freely around the axle, there's two scenarios here:


You've got super awesome bearings that have held up really well and still rotate freely.
The bearings are actually really tight and not moving at all and the wheel rotates on the axle itself.


Either way, since you mention the cones are loose, you should pull apart the hub and check the bearings. Look for dirty grease and see how the bearings/races look. Clean and repack if necessary.

New grease will always add a bit of friction, but as it breaks down and heats up (during use) it usually becomes more viscous, therefore reducing friction.