WiFi Partial Slowdown

I recently upgraded my internet connection from 20Mbps down/2Mbps up to 100/10. My Wifi connection is limited to 20 Mbps down but now gets 10 Mbps up. My wife's computer (an old iMac which is next to mine in our office) gets 100/10.

I have tried installing lilu and AirportBrcmFixup following the instructions on
https://www.tonymacx86.com/threads/broadcom-wifi-bluetooth-guide.242423/
but that resulted in an unstable and slower Wifi connection, which I fixed by using MultiBeast to...

WiFi Partial Slowdown