The hour you set the email to go out is not the hour it can go out, if you have other campaigns running and they are processed before that one, then a delay will happen. If you have a smaller server and have sent plenty emails until now and MailWizz doesn't have enough resources to do its calculations when the send-campaign command starts, then you might also see a delay.
The only way to see where MailWizz spends its time is by doing this: https://www.mailwizz.com/kb/debug-send-campaigns-command/