When I boot my Ubuntu 22.04 (all packages are up to date), my cabled network is very fast (500Mps), but after a while (an hour or two) it slows down to 10Mps without leaving a log in syslog, etc. (both upload and download).
I tried using an Ubuntu 24.04 today and it is the same. It is happening while I'm working, no hibernation, etc. I tried changing cables, but it does not change. One or two hours after rebooting my network is dead slow. Unplugging the cable, or restarting the network does not help either.
All other machines and telephones connected on my box (cable or wifi) keep their speed, so it's not the ISP nor the box to blame and clearly linked to this Ubuntu 22.04 box. After a reboot it is fast again for an hour or so.