What do you have for Use Cron Based System?
If you only have 5 emails going out per batch, that is only 30 emails an hour (but in your cron, it looks like you may have the job running only twice an hour which means only 10 emails an hour!!!). If your site is generating more than that, you will end up with a backlog of emails that aren't getting sent out and pretty soon that will result in a long delay of emails to users.
Look at Scheduled Tasks > Process Email Queue... how often is that set to run?
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