So after about 3 weeks I came back to the issue and believe it or not the course now loads very quickly. So the problem of the long loading time is gone.

I am not aware of having changed anything as my desktop was not in use when I was travelling and I also did not change anything in Moodle. So I cannot really say what the cause of the problem was.

Needless to say that I am happy it works and that I can proceed with using Adapt for developing the course.