This topic has been touched on here - Re: Do individual schedules run serially or in parallel
You will probably want to contribute or review that thread.
Please mark this reply correct if the thread answers your question.
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Tableau doesn't support cache pooling, it is user based, that means the same report will be cashed once for each user instead of caching it once and use that cache for all users. Therefore I created a separate user (service account user) and created subscription for this user. next I redirected the new subscription from this new user (service account user) to all 90 users. It is a work around so Tableau server will only sends one email subscription instead of 90 emails.