Well, here's the deal-- the slower the rotational speed of the disc, the wider the data track will be burned in. For most readers, even the narrowest track is readable.
However, for older readers, and ones with dingy or dirty lenses, the narrower tracks can lead to read errors.
If you are like me and use your equipment for literally decades, then that old CD Walkman might not like to read a CD burned at 64x, but will read one done at 10x (because that's the slowest my burner will go).