an older stable update (which thread is now closed) introduced a bug for me which seriously slows down the wifi on my notebook (which got an ath9k card).
Plasma seems to scan continuously for wifi networks, and spams the journal with messages like:
plasmashell[n]: plasma-nm: Wireless scan on "x" failed: "Scanning not allowed while already scanning"
My wifi connection has large dropouts which results in around 20% packetloss and very slow overall performance:
Ping to my wifi router: rtt min/avg/max/mdev = 1.086/62.088/226.746/71.156 ms