I tried getting my camera in a higher resolution than 640x480. I followed the FAQ and added -x ... -y ... numbers to the raspi_cam_options. That didn't change anything.
Turns out that with camera type set to auto, mjpg_streamer is still started with UVC as the input module and therefore ignores the raspi section. (and it works).
Also, it would be better to document that there are only a couple of resolutions possible near where that is important to know. Both the file itself as the "how to change it" simply document -x [a number], so I decided to use a common 1280x1024 resolution as "bigger than 640x480". Then when that didn't work I tried 1280x960 as that's precisely double the 640x480. Again a resolution not on the list, so not possible.
What surprises me though is the lag I'm seeing now. mjpg_streamer is taking 8% CPU, so not stressing out the CPU or anything. I'm seeing delays of about 2s in the feedback-stream on the control tab. (the neighboring octoprint with a logitech camera currently pointed at the same printer has more-or-less instant feedback. 100ms delay max.)