Indexes are critical (100x speedup possible). Filter with WHERE. Use covering indexes. Update statistics. Consider denormalization. Test with EXPLAIN.

Indexes provide 10-100x speedup

Covering indexes eliminate table lookups

Filter early with WHERE

Join smaller tables first

Use composite indexes for multiple columns

Partition large tables

Consider materialized views

Cache frequently accessed data

Not indexing join columns

Missing composite indexes

Outdated statistics

Not using EXPLAIN

Over-indexing (too many indexes)

Not testing with real data volumes

Ignoring execution plans

Not monitoring production performance