I’d like to offer a different perspective from Buffy’s answer, although I’m in no position to make a judgment as to which is better.
My undergraduate institution was one such teaching college. But due to the growing size of the computer science major, the required courses often had hundreds of students enrolled across multiple sections. I’ll start by answering your second question first: at my college, professors in computer science absolutely hired undergraduate students to do grading for them.
There was a strong culture for students to both grade and tutor classes they had taken before, known colloquially as “grutoring”. Students who grutored reinforced their own learning, gained experience with teaching, and got paid to boot. Pretty much all of my friends in computer science grutored at least one class each semester after their first.
This includes upper-div classes: I graded fizz buzz level programs in our intro course to proofs in our algorithms course.
Professors often would employ another aid to reduce grading time: an online grader. This was usually done in addition to having grutors. We used Gradescope. Gradescope would automate the program checking by running it against test cases. The job of the grutor would then be to award points for style and flag any invalid submissions (usually students would be offered a chance to explain/resubmit).
It’s much harder for me to answer your question regarding how much work faculty did, as I was not a faculty member. There are three primary things I can imagine a professor has to do, assuming they have grutors.
The first is to design the grading rubric and any infrastructure for autograders such as Gradescope. The former can be done while writing up the homeworks, so I can’t imagine it is too much work, especially since questions will often fall into one of a few archetypes which you can have rubrics predefined for. The latter, while a lot of work, can be student-aided. I know at least one class where a student worked (over the summer I think?) to set up the autograder.
The second is to hold grading sessions, although this is often optional for upper-div courses. The required courses at my college have grading down to a science, having been refined over decades. This is useful, since they often have hundreds of assignments to grade each week. Pretty much all of them will have a “grutor party” one day of the week for about 1 to 2 hours, wherein each grutor grades their assignments and the professor is available to answer grutor questions. For the upper-divs which don’t do this, they’ll usually at least have a weekly meeting for thirty minutes to talk about any issues that came up.
The third (like Buffy mentioned) is to hold office hours. Grutors will also have grutoring hours, but if I recall correctly, they’re only obligated to help with the homework. You’ll nevertheless want to give students a chance to talk with you directly if you’re at a teaching college.
In sum, I think that “a few hours a week,” like Buffy said, is optimistically all that was required of my professors.
I’m happy to clarify any of these points if you have questions, but please keep in mind that my perspective comes from a student grader’s view at one college. Also, I think Buffy’s (and perhaps your) class sizes are much smaller than some of the ones I grutored.
Finally, this perspective is for homeworks only. In pretty much all the courses, professors graded all exams.