Answer
Slow production queries usually result from:
- Missing or poorly designed indexes
- Incorrect cardinality estimates
- High fragmentation
- Outdated statistics
- Parameter sniffing issues
- Excessive key lookups
- TempDB contention or spills
- Implicit conversions
- Heavy sorts/hashes causing CPU pressure
- Blocking or deadlocks
Most performance issues are caused by a combination rather than a single factor.