I think it is a bit more complicated but may be somebody from Mendix could explain it better. I know for a fact that when a scheduled event does not finish correctly the whole scheduled event could stop (so no new event is triggered because the old one is still busy).
I do think you could have a problem if you schedule heavy imports this way because you probably have already one processor on a 100 procent load. This is asking for performance problems on your application.
I have seen scheduled events right after each other if the first takes more than the period.
So when scheduling every 15 minutes and processing one takes more than 15 minutes the next one is started immediatly. This can even buildup a queue, so when scheduling every 5 seconds and taking a minute just once to complete then many scheduled events get fired in a row.