Here are some performance optimization techniques that can be applied to C# code:

1. Use efficient data structures: Choose the right data structure for the problem at hand. For example, use a HashSet instead of a List for fast membership checks, or use a Dictionary instead of a List when looking up values by key.

2. Avoid unnecessary memory allocations: Minimize the creation of temporary objects or arrays, especially in performance-critical code. Prefer using object pooling or reuse objects where possible.

3. Use StringBuilder for string manipulation: When working with strings, use StringBuilder instead of string concatenation. StringBuilder is more efficient for large strings and avoids unnecessary memory allocations.

4. Batch database or network operations: When performing multiple database or network operations, batch them into a single request to minimize round-trips and improve performance.

5. Use asynchronous programming: Take advantage of async/await keywords to perform I/O operations asynchronously, allowing the program to continue execution while waiting for the operation to complete. This can improve scalability and responsiveness.

6. Cache frequently accessed or expensive computations: If a computation is expensive or frequently accessed, consider caching the result to avoid repeating the computation. This can significantly improve performance for certain scenarios.

7. Optimize loops and conditionals: Minimize the number of loop iterations by moving expensive operations out of loops if possible. Eliminate unnecessary conditionals or use short-circuiting to avoid unnecessary calculations or evaluations.

8. Use appropriate collection types: Choose the right collection type based on the specific requirements of the code. For example, use List for sequential access, LinkedList for insertion and removal in the middle, and SortedDictionary for sorted key-value pairs.

9. Profile and measure performance: Use profiling tools to identify performance bottlenecks in your code. Once identified, focus on optimizing those areas through code changes or algorithmic improvements.

10. Limit unnecessary abstraction and indirection: Avoid excessive layers of abstraction or unnecessary code indirection. Instead, focus on writing clear and concise code that performs well.

Overall, the key to performance optimization in C# is to identify the bottlenecks, measure the impact of changes, and iterate to find the best possible solution.