Performance issues causes in SQL Server

1. Buffer pool too small
2. Inadequate Disk I/O subsystem
3. I/O bottleneck
4. Missing Indexes
5. Low index usage like  poorly design indexes
6. Temp DB contention
7. Data and Log file are on same drive
8. Slow network
9. Client side performance issue
10. Query plan not updated or poor
11.If Update statistics not updated
12.Memory configuration setting not done properly
13.Lock page memory not configured with sql service user.