No, and you do not have to work out the right rate yourself. Before every background run the plugin reads what the host will actually spare — the memory limit, the execution limit, and the load average where the host will say — and halves its own rate on a small host or stands down entirely while the server is busy. Nothing is lost when it stands down; the queue waits.
You can see the readings on Diagnostics, under Server Limits.
Still stuck? Ask the developers who wrote it. We aim to reply within one business day.
