Adding indexes to a database can significantly improve performance, especially for large tables with frequent JOINs or other complex queries. However, overusing secondary indexes can lead to drawbacks such as increased storage space requirements, slower write operations, and potential performance issues. To maintain efficiency, it's essential to carefully evaluate the benefits of adding indexes against their costs and ensure that they are being used effectively. Tools like MySQL's invisible indexes allow for temporary testing of index removal without affecting performance, making it possible to weigh the trade-offs and make informed decisions about database optimization.