Zammad is cycling up to 2000 mails per channel per fetch process. It then should reload usually.
If you have regular 20000 mails per inbox, I’d have questions on what the heck is going on.
That sounds fairly high volume if Zammad can’t keep up fetching the mail.
As of now email fetching cannot be parallelized. Personally I am very sure that this would bring more harm than benefit to be honest.