Discover doesn't actually download stuff, it's just a frontend for a package manager like apt, pacman, Flatpak or Snap. The package manager would need to support a speed limit feature.
KDE
KDE is an international technology team creating user-friendly free and open source software for desktop and portable computing. KDE’s software runs on GNU/Linux, BSD and other operating systems, including Windows.
Plasma 6 Bugs
If you encounter a bug, proceed to https://bugs.kde.org/, check whether it has been reported.
If it hasn't, report it yourself.
PLEASE THINK CAREFULLY BEFORE POSTING HERE.
Developers do not look for reports on social media, so they will not see it and all it does is clutter up the feed.
Switching to slow mirror can help (if such mirror exists)
Look into Bufferbloat and SQM.
One way is to replace your router with a router with good SQM (smart queue management). You can find recommended router at https://www.waveform.com/tools/bufferbloat
On a limited connection, I'd advise not updating whilst watching a video. It's also possible to download a video and watch it whilst updating.
Beyond that it's not easy to answer - as I don't use discover, but also never do updates without paying attention, so that watching a video at the same time isn't something I'd contemplate.
Very good question and one that I can't answer. However I would check to see if your router supports QoS. If it does you might be able to ger the router to prioritize important data such as video playback, gaming, video calls etc. I've done that in my router and it made it so much easier to live with 10 Mbit.
This only works when not using a VPN, do it is not a solution for all
That depends on the setup. I use a VPN 24/7, but it's configured on the router so most of my devices are routed through the VPN at all times while at home without them having to run a VPN client themselves, and can also take advantage of DNS-based ad blocking. But yes, it's not a setup for everyone.
I think using a VPN on the router is great, but this doesnt work for mobile devices at all. Especially the situation "public wifi" is not dealt with.
What do you mean doesn't work for mobile devices "at all"? It works in many ways. If you have a VPN client on your phone, just disable it while you're connected to your home wifi.