I have the same issue since i switched from Moodle 3.5 to Moodle 3.9.3 and switching the theme, to adaptable.
What is the theme your using ? it seems that for me it goes faster when iam using other theme than Adaptable.
My course contain SCORM, Forum, Test, Certificat, ressources and some links. Also there are a lot of students matriculated, more than 2000. I dont know if it is slower because of that .. Do you know?
Also i noticed that on some courses it goes fast, and some other, it goes slow (more than 15s). And it seems for now, that the only difference between a slow and a fast course is the number of student matriculated. Even if with other theme, i dont have this issue.
If you have any ideas about how to fix it, pls let me know.