Why is it a poor proxy? Someone who really understands the concepts and has the aptitude for it will get answers more quickly than someone who is shakier on it. The person who groks it less may be able to get to the answer, but needs to spend more time working through the problem. They're less good at calculus and should get a lower grade! Maybe they shouldn't fail Calc 101, but may deserve a B or (the horror) a C. Maybe that person will never get an A is calculus and that should be ok.

Joel Spolsky explained this well about what makes a good programmer[1]. "If the basic concepts aren’t so easy that you don’t even have to think about them, you’re not going to get the big concepts."

[1] https://www.joelonsoftware.com/2006/10/25/the-guerrilla-guid...

My middle school aged child was recently diagnosed with learning disorders around processing, specifically with written language and math, which means even though he might know the material well it will take him a long time to do things we take for granted like reading and writing. But, he does much much better with recall and speed when transmitting and testing his knowledge orally. He's awful with spelling and phonemes, but his vocabulary is above grade level. For kids like him, the time aspect is not necessarily correlated to subject mastery.