Clean way to do this is with a service unit override.

1. Stop emby and restore the original unit file.

service emby-server stop

2. Verify that the unit file has the original ExecStart command:

systemctl show emby-server |grep ExecStart

3. Create a unit override to add the Nice parameter. This will open an editor where you can add Nice=-5 to the service block.

systemctl edit emby-server

Add:

[Service]
Nice=-20

5. Start the Emby server. And run top/htop to verify the new nice value.

service emby-server start

Source: Emby forums

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