Lowest delay to send mails

Hi there,
thanks for this very useful plugin!

I am wondering if there is a setting, what the minimum delay is or is it called everytime wp-cron.php is triggered? I’ve set up two cron jobs, one every minute and another one every minute with sleep 30, so in theory it should be triggered every 30 seconds. But it seems, that the mails still take 1-2 minutes to get send out (remain in status “new” in the backend), is there some kind of blocker inside the plugin, to prevent too quick executions? I need to send confirmation mails to users but want to make sure, that they don’t need to wait too long for getting it.
Thanks a lot!