What's the difference in workload between the semester and the quarter systems?
Universities that follow a semester-based calendar usually have 12 to 15 weeks of lecturing during the semester, for a total of 24 to 30 weeks per year; let's say 27 weeks, just for definiteness. Universities that follow a quarter-based calendar have about 10 weeks of lecturing in the quarter, I think, for a total of about 30 weeks per year. That's 3 more weeks of teaching.
In addition, a good part of the work teaching a course is in the initial setup and in the final wrapping-up. The week before the semester or quarter starts is usually occupied by various routine teaching-related tasks, and, after the course ends, it takes about two weeks before the final exam is done, graded, and final course grades are turned in: that's an additional 3 weeks per semester/quarter, so those who are using the quarter system have an extra 3 weeks of that stuff during the year.
3 more weeks of teaching and 3 more weeks of "stuff": that's an additional 6 weeks spent on teaching-related activities - 39 weeks per year instead of 33. For professors, going from the semester to the quarter system is an 18 percent increase in teaching load.