Uups, sorry. Problem solved.
Missing "NodeBB is rebuilding front-end assets" was the problem.
There is the traffic management settings and the option to set the threshold for the event loop lag. Under the value you can set there is a short help text that confuses me a bit:
Lowering this value decreases wait times for page loads, but will also show the "excessive load" message to more users.
Why does lowering of this value decrease wait times for page loads? I went through the code and I didn't feel like this value would change anything in regards to the page load time. What am I missing?