If the student tries to read everything carefully at a leisurely pace, that’s going to take 25+ hours a week of just reading.
History students can’t afford the time to read everything line by line. They need to learn to skim, track the structure of the argument, figure out which bits are fluff and skip over those, and then focus down on the tricky sections.
It’s not that close reading isn’t also important in those fields (it absolutely is!), but not everything can be read closely.
To the grandparent poster: 3000 words is peanuts compared to the kind of reading loads this professor is talking about.