[SOLVED] Download speed very low, upload speed OK

[jakenl]

Today I finally linked the one problem with a second one.
Last week I realized that a sudo apt-get upgrade was terribly slow, sometimes 10kb/sec or nothing at all. I blamed the server for that and hey, it's free, so don't complain.
However, an add-on uitzendinggemist.nl which downloads Dutch TV from the public media didn't work either. the log file is spammed with buffer issues etc. So I copy/pasted that log in the github of the developer.

Today I figured, what if the download speed is so bad, that the plugin, which streams from the internet, can't load the stream properly? I downloaded speedtest-cli and indeed: download 0.5Mb/sec and upload 90Mb/sec.
I have 2 RPI's and the other one, running raspbian has 70Mb/sec down and 90Mb/sec up.
I switched network cables and plugged the one from my pc in. No difference
I put the SD-card + USB with OS into my previous RPI2, instead of the current RPI3: no difference.
In the system panel I found the download limiter. It was set to none, and I put it to the max value, by clicking on the up button: no difference
(I rebooted the RPI between several trials and even rebooted the router once)
The slow speed goes with the OS, change of hardware (device/network cable) make no difference. What on earth can have triggered this?

EDIT
One more thing to try:

Code:

sudo ethtool -s eth0 speed 100 duplex full autoneg on

Unfortunately this didn't help.
I guess I will put a 1 month old image back and see if that brings back the expected speed.