Why while one XHR request hangs in pending, requests to get images from CDN also become pending?

Question

JavaScript

There is a third-party script (tracker)
We insert it into html
It loads another script
That script makes a request that hangs in pending up to 504
While pending hangs, pictures (pending) from the CDN are not loaded, which is in no way connected with the script
Play artificially does not work, the problem appeared only once on sale

Answer 1

The browser has a limit of 5 threads per host. I can’t imagine how 5 turned out from one and hooked as you say another host / subdomain.
If only some browser extensions were superimposed. Seo tools or something else was not at that workplace?