1. The sequence of fields must be same as per database table
2. During writing select query write all fields in sequence as per database table.
3. Never write select statements inside loop. endloop.
4. Use st05 SQL trace, se30 run time analysis, code inspector, SLIN (Extended Program Check),etc.
5. Use select single * statement instead of select *
6. Always use primary key
7. Use binary search but before using binary search sort that table.
For more details on perfornamce tuning check 10 best ABAP programming standards for performance tuning