Yeah there are slow pokes and races horses in the fleet...most guys won't notice unless they are sort to sit up there and pull out the book out of curiosity.
I flew one plane that was 17kts faster then book at FL410...translates to about every 5 hours I flew, the other guy in the same plane was 100 nm behind me... at the end of the year in a busy flight department...he's sitting up there 50 hours a year longer...translate that to duty time...that's about a month.....which has all sorts of effects on maintenance, down time, extra stops, extra brake wear, not being at home, the length of the days, time to climb...
Then you have the dogs of the fleet that are say about 5% on the back side of book...combine that with guys who can't flight plan efficiently....the numbers really get stupid...