Even with the realtime kernel, following the steps here: https://jackaudio.org/faq/linux_rt_config.html and setting jack to realtime (QJackCtl -> Settings -> Parameters -> Realtime, or using the
jackd -R command line arg), I was getting a ton of XRUNs (visible in QJackCtl as the red number, or in the Messages dialog), making the sound stutter a lot.
Increasing the Frames/Period and Periods/Buffer made the XRUNs go away, but increased the latency to a few hundred ms.
It works on a different computer with a similar manjaro installation, so I guess it might be somewhat related to the hardware. The mainboard is a "MSI B450M Mortar Max", it's an AMD system.
aplay --list-devices says Realtek ALC892,
lspci -v | grep -i audio says "Starship/Matisse HD Audio Controller".