Hey!
This has been plaguing me for some time and wanted to share a solution.
I had 2 computers a desktop (wired connection) and a laptop. Both Fedora. Now, on the desktop, some software was nearly unusable, with network requests often taking several minutes: GNOME Software, Weather, GNOME Web, GNOME Maps…
On the other hand all other software worked fine with 1000Mbps.
After a long debugging session looking around resolvectl settings, DNS settings, debugging packets with Wireshark, it turned out that in some apps, like GNOME apps, instead of the DNS requests I would expect, some wpad DNS and even LLMNR queries have been going on.
I was sure that this was due to some network setting!
It turned out that the “network proxy” in GNOME was set to “automatic”, causing these wpad proxy auto-discovery queries, timing out in several but not all applications. 
This was super confusing! Do you know why it happened in certain apps only?