Mongo database design and scalability

Hello all,

I was wondering if you could please advise me on a mongo database design,

Currently we have a Xamarin app using realm and Laravel app using the mongo driver. There are multiple companies sharing a singular cluster with their data being split by partition sync which uses the unique company ID.
This allows the correct users to access the correct data. The data will be kept in the shared cluster whilst the realm access is required but once the data is historical it will be moved to a companies own cluster to help release space on the shared cluster.
This historical data can still be accessed from the Laravel web app but not the Xamarin app.

My worry is to do with the scalability of the cluster and the current design of the database. Will a cluster slow down after it has millions/billions of documents? Even if the database is sharded and indexed?

Thank you so much for your help! :smiley:

kind regards,