100,000 Internet line and still lags (PC)?

We have a 100k line and if i play rainbow online, everything runs smoothly. Now I have three cohabitants and if at least two of them are at home, then I always have lags. How can that be? When I lived at home with my parents in the countryside and we had a 16k line, there was little to no problem, even though my parents and brother were connected to the phone / tablet.
My roommates do not play, do not stream anything and watch all but Netflix…
How can that be?
And, who does not care about it: how does the whole system basically work, so how are the 100k split? → 4 user → each user 25k? Or: If user 1 requires only 10k, does user 2 receive the remaining 15k in addition? Or completely different?
Whoever replies: Thank you my best / my best:]

Do not stream anything

watch […] Netflix

You contradict yourself.

You can try to prioritize your computer on the network so that your game's data packets are placed above everything else. Take a look at the workload of your network, then you might see what the problem is.

The allocation of the throughput always works individually, which is not easy to describe. Basically, video streams etc. Are prioritized by standard routers.

Come to the 100k because ever with you, have you verified times on speedtest.net? And are you connected via LAN or WLAN?

Ahh is wrong, I meant that they do not stream anything in the sense of uploading. Whether that makes a difference I do not know, but that does not matter now.
I'm connected to Wi-Fi and it comes so ~ 85k, but that's still a lot.
"You can try to prioritize your computer on the network so that your game's data packets are better than anything else, so check out your network's load and see what the problem is." → Thanks for the tip, I'll deal with it!

Does the 85k also arrive when your roommates stream? This is generally the case over Wi-Fi because cheap routers and clients are not able to send multiple devices at the same time. I mean, they always look one after the other: a package to device one, then a device to package two, etc. This can lead to heavy power losses. If you somehow have the opportunity to connect via LAN, do it. Alone to find out if it's because.