The answer I wanted was [this one](https://stackoverflow.com/questions/7635515/how-to-set-cap-sys-nice-capability-to-a-linux-user)

Anyway
```bash
sudo setcap 'cap_sys_nice=eip' /usr/local/bin/renoise-3.4.2 
sudo setcap 'cap_sys_nice=eip' /opt/REAPER/reaper
```
then you can check
```bash
$ getcap /opt/REAPER/reaper 
/opt/REAPER/reaper cap_sys_nice=eip

$ getcap /usr/local/bin/renoise-3.4.2 
/usr/local/bin/renoise-3.4.2 cap_sys_nice=eip
```

Semi-relevant: There is [this stackoverflow question](https://stackoverflow.com/questions/8887531/which-real-time-priority-is-the-highest-priority-in-linux) and [this unix.stackexchange question](https://unix.stackexchange.com/questions/605378/how-to-start-a-realtime-process)

See [this linuxaudio article for info on CPU frequency scaling](https://wiki.linuxaudio.org/wiki/system_configuration#cpu_frequency_scaling) which you don't necessarily want.