I have RPi 3B+ connected to exactly the same router as my PC (wired!).
This isn't "today's a bad day" because I am observing such deviations for days, if not weeks so far (anyway, as long as I am measuring this).
Can someone explain me this quite odd behaviour?
EDIT: Both PC and RPi are wire-connected (so no Wi-Fi related issues are asked here).
EDIT: Someone could think that this is a question about 3rd party software (speedtest-cli) and thus off-topic. It is not. The very similar results (very low download speed in compare to overall network speed) I am getting in many other places and when fetching data from many other sources (i.e.
apt upgrade for updating packages and
wget for fetching files from various sources). This question is about RPi hardware (RJ45 port and chip that controls it) and its quality, performance or speed.