Origin of "slow on the uptake"
Uptake (n.) meaning ability to understand is from the beginning of the 19th century. There might be an analogy with the later meaning that refers to a "pipe leading up from the smoke box":
"capacity for understanding, perceptive power," 1816, from up (adv.) + take (v.). Compare Middle English verb uptake "to pick or take up" (c. 1300).
Meaning "pipe leading up from the smoke box of a steam boiler to the chimney" is from 1839.
According to Ngram the expressions "slow/quick on the uptake" are both from the early 20th century.