My school, a community college in California, looked at this sort of thing about 10 years ago when the union was pushing to shorten the semester from 18 weeks to 16 weeks. We have also historically had 6-week or 8-week summer classes, and many faculty have always been skeptical about the quality of instruction in those classes. Often when I see a student who doesn't seem to have the knowledge covered by the prerequisite for my class, I check their transcript and see the same pattern. They took the class once or twice and failed it, then took it a third time in a short summer class and passed.
At the time when we were discussing the shortening of the semester by two weeks, the supporters of the proposal went around saying that "studies" showed that success was actually higher with the shorter semester. Later it turned out that these studies never actually existed.
The problem with trying to do a study of this is that you can't control for all the variables.
The student population taking an 8-week summer course is a different population than the one taking a 16-week course during a regular semester. For example, we would get a lot of students from UC who would show up during the summer at our community college in order to take care of their one remaining requirement.
My school finally did shorten the semester by 2 weeks. You would think we could look at success rates before and after, and see if there was a change. But most faculty responded to the change by cutting material and assigning less work, so again there is an uncontrolled variable.
For a lot of faculty who don't care about their jobs, an 8-week online course is the greatest deal ever. You've already recorded your canned videos. The homework is in the textbook publisher's online system, so you don't have any grading. You were always doing zero work, and now you're just doing zero work twice as fast. Then in November you take that trip to Costa Rica.