I have a pretty stable and fast enough internet connection so I rule this out.
I use Firefox 63.0 on a Ubuntu machine.
When I click "x questions with new activity" and the x happens to be some integer in the 30s or higher it takes really long until the new questions are displayed.
Now when I click just "Questions" on the left it loads in an instant.
Does this maybe have something to do with the fact that I display 50 questions per page?