Not entirely sure, but you may end up with the same forum message being sent more than once if the mailout takes more than 5 minutes to exec. The mailout code is quite conservative, but I suspect that there are a few race conditions that your sleep(6) will make painfully evident.
Possible workaround: have a lock on cron runs, so they never overlap -- take a look at dotlockfile. But bear in mind this has its own problems too.
Hardware and performance
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