Indexing key fields used in report criteria can speed up the query execution and reduce the report run time. Indexes can be created by Salesforce automatically or manually by request. Creating one skinny table per report can also improve the reporting performance by storing frequently used fields in a separate table that does not include complex formulas or joins.

Using a validation rule is the best way to ensure that all customers provide either a phone number or an email address when registering for an account, as it allows defining a formula that evaluates whether a record meets certain criteria. Using required fields may not work if both phone number and email address are required.
Using an Apex class or a Process Builder may be more complex or cumbersome than using a validation rule.