Recently I've noticed that internet speeds on my MBP 2018 (Mojave 10.14.4) are decreasing with time. For example, after a day it will go from 100 MB to 60 MB and it will continue to decrease. The plot below shows download speeds sampled every 15 minutes over the course of ~3 days.
A few things:
This is not my ISP, since it works perfectly on other devices.
After restarting the computer, it goes back to the maximum speed and countdown starts again.
It is both for wi-fi and ethernet.
Logging out and back in doesn't fix the issue.
There is no software that runs in the background and utilizes that bandwidth.
Network diagnostics don't show anything.
iperfbetween Mac and PC over Ethernet and WiFi, gives same low numbers as on Mac. Speedtest on PC is OK.
Running Speedtest from Parallels Ubuntu VM also yields low download speeds.
For Ethernet, I am using Belkin HD dock.
Would appreciate any help or guidelines on how to debug this issue.