I reinstalled my Kubuntu 18.04 system 3 weeks ago. I have installed updates in the last few days, then the internet speed has dropped significantly. Instead of 250/20 Mbit, only 20/15 Mbit can be measured. For USB Live system does not have this phenomenon.
On the live system, the 4.15.0-20-generic kernel runs while the currently installed 4.15.0-36-generic kernel is running, trying to boot with the older kernel in GRUB, but the situation did not change
uname -a: (live) Linux kubuntu 4.15.0-20-generic #21-Ubuntu SMP Tue Apr 24 06:16:15 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
uname -a: (installed) Linux adam-desktop 4.15.0-36-generic #39-Ubuntu SMP Mon Sep 24 16:19:09 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
Motherboard: Asrock 960GM-GS3 FX CPU: AMD Athlon II X2 250 3.00GHz AM3 NIC: TP-Link TG-3269 (Realtek RTL-8169SC chip)