A Span<T> cannot cross an await boundary
Keep span work between awaits. Carry memory or its backing buffer through asynchronous I/O.
Async
await foreach does not automatically use your cancellation token
A CancellationToken in scope does nothing unless the async enumeration receives and observes it.
Use async for I/O, not as a substitute for CPU parallelism
Use async to avoid blocking while I/O is in flight, and treat CPU parallelism as a separate measured design decision.
Use ValueTask only when you know why
Prefer Task unless you have a measured allocation reason and a simple consumption contract.
Do not hide exceptions inside fire-and-forget tasks
If work can fail, give it ownership, logging, cancellation, and a lifecycle.
Start independent tasks before awaiting them
Create unrelated asynchronous operations first, then await them together.