It is almost too simple to blog about, but on the other hand, it caused me some grief.
Short summary: fixing the WLAN channel fixed the WLAN performance. It had never occured to me to try switching the channel because signal strength had always be shown as excellent. However, apparently the external monitor somehow disturbed the connection on the default channel.
I had been experiencing really bad network problems on my home network for a while. It had gotten so bad that I even used Tethering on my Android phone as a fallback at times, and I did not even dare to play my beloved Carcassonne games on Brettspielwelt anymore, because they would be so painfully slow.
Disconnecting my MacBook from the external monitor and going next to the WLAN router with it seemed to yield big improvements. So it looked as if the signal strength simply was not good enough. I already thought about buying another router or even plugs for networking via the power grid. However, the signal strength indicator for the WLAN was always at 100%, which seemed strange.
As a last test, I remembered about the command line tool ping. pinging my router, ping told me I had 20 to 40% packet loss. Really bad. But then I disconnected the monitor and tried ping again - suddenly the packet loss was 0%. After some more trials, like connecting the monitor to another computer, it became clear that it is really when the external monitor is connected to my MacBook that the WLAN connection goes bad. Apart from ping, using some of the speed detection web sites also was informative. Download speeds quadrupled without the external monitor being plugged in. By googling I found that others experience this, too.
At first I was frustrated, expecting costly repairs or hardware replacements, but then I read somewhere that changing the WLAN channel might help. So I switched my router from automatic channel selection to channel 5 (before it was using 11), and since then, the WLAN connection seems to be fine. (Knock on Wood - it's only been 30 minutes since I changed it).
So that's it - switching the channel should have been the first thing to try when I experienced connectivity problems. However, it didn't occur to me because the network strength indicator always showed a strong signal. Only with ping was I able to see the packet loss despite of the good signal.
It's such a relief to have a snappy network connection again. Especially as I didn't have high hopes for finding a solution by myself, as I know next to nothing about optimizing WLAN networks.