What range of rotational speed for bike wheel bearings?
Searching for "ndm rotational speed" finds that D and d are outer and inner diameters of the bearing. n is not the usual symbol for speed, but neither is the final unit "millimeters per minute divided by pi" an usual unit. And here we go: 

A slightly optimistic approximation for road bike speed would be 10 m/s (36 km/h, 22.5 mph) and wheel circumference 2 m. That gives us roughly 10 m/s / 2m * 60 = 300 rpm. The bearing track diameter is around 50 mm (slightly under 2 inches and yes, the sum divided by 2 is the average of inner and outer diameters). 

All together this gives us 300 rpm * 50 mm = 15000 weird units. This is very well inside "very low" category for roller bearings, and you can double the speed or halve the wheel size before you reach the "low" category. For ball bearings, there is no "very low" category, and you need to exceed the approximation by factor 6.66 before you break out of "low" category. You can change all numbers involved in the calculation by quite large amounts, and the result will always stay "very low" or in extreme cases "low".