Oh, OK, I thought it was every Monday morning.
In fact, I would rather have the weekly digest every Monday morning for everyone, it's a bit strange that everybody does not receive the weekly e-mail the same day.
For performance issues I'm not really worried, it's a batch job anyway. And E-mails are sent with the Async annotation, which means they will be sent in the background, one after the other.
Maybe you can add some logging, so we can check in the logs if there is some potential issues? I'm really not worried about this at the moment, but maybe I'm wrong.