It's more likely he meant to
slow down the saving of logs, not speed it up.
mIRC (presumably) keeps the log files open all the time, which can cause file corruption if the system is not shut down properly. This was also given as a probable reason for
mirc.ini corruption.
If mIRC saved logs at 5 minute intervals then you'd lose a maximum of 5 minutes worth of logs rather than the entire contents.