I have also seen the "Set Details" page as loading very slowly, and may have found a cause and a workaround.
After some research, I suspect the problem is a fix made to the code which searches for possible problem header files to display as choices on this page. The corrected code now will search under all symbolic links in the course template directory, unless the links are included in $courseFiles{problibs} (which by default only includes Library, Contrib, and capaLibrary). In the past, it seems that the search would not actually search under symbolic links. In my case, I had relatively large local problem collections available via symbolic links. Delays in this search could also occur if there is any large directory tree under the course templates directory.
In my case, I seem to have gotten a significant improvement in performance of the Problem Set Details page by adding the two additional symbolic links being used in my courses to $courseFiles{problibs} by setting a local value for this in localOverrides.conf. It can also be done in course.conf for a specific course.
However, a quick hack to see if the search for header files is the cause of the slow loading of the Set Details page would be to edit lib/WeBWorK/Utils/Files.pm (under webwork2) and temporarily modify listFilesRecursive so that right after it defined @matches it would just return
return @matches;
so that no search will be run and then restart the server.
According to the commit message in GitHub a quick search, the listFilesRecursive routine is only be used for this specific search, so it should not break any other functionality while you are testing it.
https://github.com/openwebwork/webwork2/commit/567b80822165d1956a9260e06cdf73a0a0de1afe