How can I calculate the amount of time a learner spends on each slide?

To do this accurately you'll need to keep track of progress in case the user goes backward, you don't want the time to get over-written.

You'd need to use some JavaScript and the onEnterSlide listener and put the time in an array if the slide has not been visited previously. You can write the SCORM variable cmi.session_time when you enter a slide. The only issue would be if the user did go back the time on the slide where they went back from would not be accurate without some other coding to determine how much time is spent on previously visited slides.

You could store this in cmi.comments or suspend_data or even a google sheet.

We have already done something similar storing Course ID, Name, Pretest Score, Quiz Score and date to a google sheet.

Maybe have a look at this old post. It is not exactly what you want, because you'll have to store the result for each slide in a user variable. You'll only see how you can use available system variables. However, not sure it will be possible to report those values, depends on the LMS:

Display Time information - Captivate blog