Transactions and Read/Write Concerns are crucial for achieving performance at scale across MongoDB applications. Single-document atomicity provides guarantees that meet data integrity needs, while multi-document ACID transactions offer a complete range of use cases with MongoDB. To maintain predictable database performance, developers should consider transaction runtime limits, number of operations in a transaction, and distributed, multi-shard transactions. Exception handling is also essential to catch and retry transactions that abort due to temporary exceptions. The choice of write guarantees and read concerns can significantly impact performance, so it's crucial to choose the right ones for each application. By following these best practices, developers can ensure their MongoDB applications are optimized for performance and scalability.