feat(perf): basic C# performance optimizations

[currantw]

Describe the feature

Consider basic, well-known C# performance optimizations across the codebase to improve performance.

Proposed Solution

Possible optimizations (non-exhaustive):

• Memory and allocations

  • Use Span<T> / ReadOnlySpan<T> / Memory<T> (see feat(performance): Adopt Span<T> and Memory<T> #204).
  • Use stackalloc instead of new byte[] for small, short-lived byte buffers.
  • Use ArrayPool<T>.Shared or MemoryPool<T> for larger recycled buffers.

• Async and concurrency

  • Avoid LINQ (allocates enumerators and closures); prefer plain for / foreach loops.
  • Use string.Create() or pooled StringBuilder instead of string concatenation in loops.
  • Prefer ValueTask<T> over Task<T> for methods that frequently complete synchronously.
  • Use ConfigureAwait(false) to avoid unnecessary synchronization-context captures.
  • Avoid async on methods that simply return an already-completed task.

• Collections

  • Pre-size List<T>, Dictionary<K,V>, etc. when the count is known.
  • Use FrozenDictionary<K,V> / FrozenSet<T> (.NET 8) for build-once, read-many lookup tables.
  • Use Dictionary.TryGetValue instead of ContainsKey + indexer (single lookup vs. two).

• FFI/interop

  • Use [SkipLocalsInit] on hot interop methods to skip zero-initialization of locals.
  • Prefer nint / nuint over IntPtr / UIntPtr for cleaner pointer arithmetic.

• General

  • Seal classes not designed for inheritance so the JIT can devirtualize calls.
  • Use static lambdas to avoid closure allocations when no state is captured.
  • Use the ThrowHelper pattern — move throw statements into separate non-inlined methods to keep the happy path compact (consider for GuardClause).

Acknowledgements

• I may be able to implement this feature request
• This feature might cause a breaking change

Related

⚪ None

Additional Information

• Microsoft's performance guidance and the high-performance .NET docs cover most of these in detail.
• feat(performance): Adopt Span<T> and Memory<T> #204