As tables grow, the difference between a query that returns instantly and one that crawls for minutes usually comes down to indexes. An index is a separate data structure that lets the engine find rows by a column's value without scanning the entire table, much like a book's index lets you jump to a topic without reading every page from the beginning.
Indexes are the single most important performance tool a beginner can learn to wield, but they are not free. Each index speeds certain reads while slowing every write and consuming storage, so indexing is a deliberate trade-off rather than something to apply everywhere. Knowing which queries benefit, and which indexes actually get used, is what turns indexing from guesswork into engineering.
Equally important is learning to read what the engine is doing through EXPLAIN, which reveals whether a query uses an index or scans the whole table. Combined with understanding when a condition can use an index and when it cannot, this lets you diagnose slow queries methodically and apply the right index, the foundation of practical query performance tuning.