I've done a timed 45 minute event at an average speed of 18.6 mph, (twice) on my 46-18 (68 gear inches) with NO drafting. This meant an average 92 rpm cadence. A higher gear would have been slower for me. YMMV
I understand that everyone's preference differs, but I'm convinced that many, maybe even most SS/FG riders are over geared, and can often be found lugging that gear at relatively low cadence. (and speed) Most riders are not averaging 20+ mph.