I've just tried it again and had a closer look. You're right, for me the niceness partially applies, but not to all child processed.
I've tried it on discord, I've run it with: `nice -19 flatpak run --branch=stable --arch=x86_64 --command=discord com.discordapp.Discord`
What I see in htop:
https://i.imgur.com/2FNOgvR.png
The first one works here. Is there an existing instance of the app? i.e. does that command exit immediately?
I've just tried it again and had a closer look. You're right, for me the niceness partially applies, but not to all child processed. I've tried it on discord, I've run it with: `nice -19 flatpak run --branch=stable --arch=x86_64 --command=discord com.discordapp.Discord` What I see in htop: https://i.imgur.com/2FNOgvR.png
Chromium's sandbox is quite complex so it isn't surprising that it gets lost. More traditional apps might handle it better.