From systemd README:
We recommend to turn off Real-Time group scheduling in the
kernel when using systemd. RT group scheduling effectively
makes RT scheduling unavailable for most userspace, since it
requires explicit assignment of RT budgets to each unit whose
processes making use of RT. As there's no sensible way to
assign these budgets automatically this cannot really be
fixed, and it's best to disable group scheduling hence.
Disabling it allows apps like JACK to run with real-time scheduling.
It's disabled in Debian's and Arch's kernels as well.