I think the "average" speed for domestic QSOs is closer to 17 WPM (here in the USA, anyway). You'll hear some in the 20s and beyond, you'll occasionally hear some at 5 WPM, but the majority are between 14 and 20.
Most tests and DX are 30+ WPM but that doesn't mean you need to be proficient at prolonged copy at those speeds to work it. You just need to be able to copy callsigns at that speed, even if you have to listen a few times.
My comfort zone for QSO is maybe 18 or 20 WPM, maybe even slower if it is a long QSO, but thanks to RuffzXP I can copy call signs at 40 WPM. But, I get in to trouble fast if they want to chat that fast, which wasn't very often when I was active at it