Improve mongos performance with jumbo size of roles assigned to single user

In large multi-tenant environments a lot of customers will create 1 DB per tenant along with a user for that tenant. When the app or other users that need elevated privileges to multiple DBs, it could be assigned separate roles for every database. This is due to GDPR compliance use cases, separation of duties generally dictates that no one user can have access to all data.

This ticket is to find a solution to improve performance of such accounts