A lot of skaters go around bragging about how much faster their new ABEC 7 bearings are than their old ABEC 3's. This may indeed be true but it has nothing to do with the ABEC rating itself. Not unless you are skating at 330 mph, that's based on a 608 bearing limiting speed of 32,000 rpm. Regardless of how fast you plan to go, speed is affected first and foremost by the choice of lubricant. The two most common lubricants are grease and oil .
Grease is basically oil with a thickener or soap.
The thickener acts like a sponge to soak up the oil when not in use. It keeps metal parts from wearing against one another and dirt away from the sensitive inner workings. Grease Oil prevents material wear prevents material wear requires little servicing requires frequent servicing increases torque has low torque lasts a long time does not last as long helps keep dirt out can suspend contaminants susceptible to dust and contamination From the above you can see that a greased bearing requires little servicing yet cannot run as fast as an oiled bearing. As grease is thicker, it acts as a seal against dirt, but at the same time it can increase torque and slow down the bearing. Higher rated bearings are more expensive because they use better materials, period, at least as far as skating is concerned.