DP dedicated renices itself to (hard coded) 19 during level changes
If you have a cpu bound job such as a compile running at default priority on all cores, this can cause DP to take so long to change levels that all clients have timed out by the time it has loaded the next map. Setting a high priority at runtime does not prevent the renice to 19.
I guess the purpose of the renice is to prevent DP level changes causing lag for other game servers on a single core server.
Ideally this would be configurable with cvars instead of hard coded in sys_shared.c. Then admins could give DP a high priority during gameplay and have it renice itself to default priority during map changes, if required.
Workaround for admins: use the -nonice argument (thanks malice) or run DP at a realtime priority (less than 50 for single core servers!), as Linux ignores nice levels for realtime processes. On a single core server, this will lag your ssh, http etc during map changes.