Question about download speeds degrading

So most of the time my internet looks like this:

But after a certain amount of time (12-24 hours?) it flips over to this:

It's a pretty consistent 90% drop in download and upload speeds after a while. This is all on the built-in NIC on the motherboard. A reboot will see the download speeds go right back to normal so it doesn't seem to be an issue with the router or the modem. Curious what this could be in the system itself? It's not terrible to reboot every once in a while (and the speeds are generally "fine" even when they're slower than usual) but I'm curious if anyone has any ideas as to what might be causing this.

Hmm.. Is there some sort of 'Airtime Fairness' (ATF) setting on your router or modem that decides to assign less network resources to your PC as you use it for many hours?