Not disputing that some did this, but it was bad practice. Lapping in a cheap hub's bearing might help, but decent hubs were already too good to benefit. Replacing grease with oil lowers parasitic drag caused by grease's viscosity, and is fine for the track and road races in dry weather, though it shortens the service interval.

This brings us to the third item, running bearings loose. While it helps an unloaded hub spin better, it actually increases friction in real world conditions. Then hubs that allow user service use angular contact bearings. When loose the bearing surfaces and balls are not in constant contact all the way around. This allows the axle to push itself up between adjacent balls at 12 o'clock driving the balls around and into each other where they rub as they spin in opposite directions. The lowest friction mode for an angular ball bearing is at the minimum preload which ensures zero play. Slightly tight has lower friction than slightly loose.