Today, I assembled a PC using the MSI B450 Gaming Pro Carbon AC as its motherboard. This motherboard comes with wifi. Already before installing Kubuntu 18.04, I noticed that the downlink is terribly slow (highest about 1 Mbit/s, often way below). This stayed the same after booting into the installed operating system.
The wifi's uplink is not affected. It is much faster than the downlink. When I connect my computer and my laptop via a LAN cable and share the laptop's internet connection (same wifi network) with the computer, it has a fast downlink too.
I connected a cheap old wifi stick to the computer. The wifi stick is so cheap and old that it is quite limited itself but even it pulls data with over 60 Mbit/s.
Kubuntu does not list any additional drivers to install.
How can I improve the speed of the on-board wifi?