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Exim spawns a new queue runner every n minutes as configured by the argument to -q; up to queue_run_max can be active at the same time. Spawning a queue runner only every 30 mins means that a message that failed delivery on the first attempt (e.g. due to greylisting) will only be retried 30 minutes later. A queue runner will immediately exit if the queue is empty, so it is more a function on how quickly Exim will scale to mail load and how quickly it will retry than something that is taxing on an otherwise empty system.
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at https://nixos.org/nixos and in the manual in doc/manual.