This is a fairly broad topic, to be honest. What version of Qlik Sense Enterprise are you testing on? I ask because there is a changeset in June 2020 which is very relevant here:
Improved performance for resolving linked master objects in OpenApp Jira issue ID: QB-1426 Description: When resolving linked master objects in sheet objects, OpenApp exhibited poor and undue performance scaling with the number of sheets and sheet objects present in the app. This has been resolved and the proper object indices are now used in this lookup, taking practically negligible time
The crux of this issue was inefficiencies when opening an app with a lot of sheets with master items used in those sheets. This would match the pattern that you have here.
The other one you've already isolated as not at play is very broad security rules can cause slowness (i.e. a user who can see all user's personal sheets on published apps).
Thank you for your help, and sorry for the delay I wasn't at the office.
We are currently on QS June 2020 Patch 2.
That's still a good idea to test removing master objects/dims/measures or other content as we also have quite many variables / bookmarks in these apps. Maybe we could eliminate a few latency potential causes from our test.