I measured speed by various tools (speedtest-cli, speed tests in browser…) and the results are consistently low. (And the internet is slow objectively. Loading of any page is visibly slower)
Everything is connected via ethernet cable directly connected to router.
Ok, then it is probably not a saturated frequency band.
Try to look at what log options you have on the router.
You need to figure out more about the slowdown, like what packet types, IPs, ports and so on that the router is busy with.