Important facts which needs to be considered while designing any solution which includes Large data volumes in Salesforce

• Sharing calculation is even performed for records which are in recycle bin which may impact your performance. So if you don't need records then perform hard delete on records after archiving it.

• Only Bulk API supports hard delete functionality.

• Adding custom indexes on records may improve query performance but can degrade the performance of database insert and update. That is reason that Salesforce doesn't allow developers to add custom indexes by themselves. You have to raise support ticket and Salesforce will evaluate your use case before adding custom indexes. So always evaluate the pros and cons before adding any new custom indexes.

• As a best practice, don't use formula fields in where clause in SOQL. It degrade the SOQL performance if it doesn't return deterministic values (formula returns pre-defined set of values which are repeatable).

• Now Salesforce allow you to add custom index to formula fields only if it gives deterministic results

• Use readonly attribute on VF page if you want to query more than 50 millions records and display the result on UI by processing those records.