I rode thousands of miles on 49-46 chainwheels, and I'm probably in the thousands for my 45-42 current chainwheels (I'm getting weaker with age). It's pretty flat around here, and sometimes all I need for comfort is a 6%-7% difference. Shifting fast takes skill, but 'weaker' means 'slower.' Speed is relative, of course. More to the point, speed for me is a memory....