@Maciej That's not rude at all: good question.  The short answer is, yes.  We have three "power users" in our household.  Two live stream and game (Twitch) and me: I'm a software developer.  All three systems are wired ethernet.  With QOS off, bufferbloat is so bad that if someone downloads a large file like a 100GB game, the other two basically can't do anything.  Ping goes up from the normal 10ms to over 200ms with some spikes up to 2000ms.  Bufferbloat tests during those times via the online test tools reports an "F" for bufferbloat.  Turn QOS on and you can't even tell someone is downloading a large file.  Ping may go up from 10ms to at most, 15ms but we can all continue doing what we are doing.  With QOS on, I get an "A" rating for bufferbloat while downloading a large file.

So simply put, our internet connection is unusable without QOS.  It's fine until someone starts downloading a big file but at that point, it's bye-bye internet.

Mike

@Mikey_C Well then, there's not much I can say. Apart from testing another brand, all I could recommend is setting up a daily reboot, but I'm sure you've thought of that already ...

I myself no longer use the C5400X, as I've switched to the Archer AX11000. Indeed, I've tried to replicate your issue on this unit, however no go - even though I've given preference to games (my younger daughter is a great gamer), and I have a 940/60 connection, the problem just won't appear. Maybe the AX11000 firmware is better written, or perhaps my ISP and modem don't pose such a challenge, as I've read on SNB that these two elements can also be the cause of the problem.