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.