An API that cruises through staging can crumble the moment real traffic hits. The failure is rarely dramatic. It is a death by a thousand cuts. A blocked thread here. A redundant query there. Under light load, these small inefficiencies hide. Under production pressure, they compound into thread pool starvation, database choking, and response times that destroy user trust. Performance tuning is not about finding one magic fix. It is about building a system where every layer respects the scarcity of threads, memory, and database connections. When you treat those resources as finite, you stop reacting to outages and start preventing them.
Kill Sync-over-Async Before It Kills You
The single most destructive pattern in high-traffic ASP.NET Core applications is sync-over-async. You see it when someone calls .Result or .Wait() on an asynchronous method because they need the value now and do not want to refactor the call stack. That decision blocks the calling thread. The thread sits idle, waiting for work that is already happening elsewhere, but the runtime cannot reuse it for another request.
When enough requests do this, the thread pool starves. Your CPU graph looks healthy because the processors are not busy, yet your latency explodes. Requests queue up, waiting for threads that will never free up. The fix is mechanical but requires discipline: use await all the way up the call stack. If a method calls an async API, it must be async itself. There are no shortcuts here. Every synchronous blockage you remove buys back headroom for real traffic.
Stop Wasting Work on Disconnected Clients
Clients disconnect. Browsers close tabs. Mobile apps lose signal. If your server does not know the client is gone, it keeps running database queries, parsing JSON, and burning threads on a response nobody will receive. Pass a CancellationToken into every async operation that supports it. That means Entity Framework queries, HTTP calls with HttpClient, and any long-running background work.
When the connection drops, the token triggers cancellation, and the work stops immediately. This preserves database CPU cycles and returns threads to the pool faster. It is a small change in method signatures that pays off under load.
Cache What Does Not Change
Your product catalog probably does not change between every request. Your configuration flags certainly do not. Yet many APIs hit the database repeatedly for the same static data. Output caching in ASP.NET Core lets you store rendered responses and serve them directly from memory without touching your controllers or database again.
Use tag-based invalidation carefully. When you update a product, invalidate only the tag associated with that category or item. You do not need to flush the entire cache. This keeps your cache hit ratio high and your database query count low.
Fix Your Data Access First
Database calls consume the bulk of request time in most APIs. Before you optimise anything else, look here.
If you are querying data only to display it and never planning to update it, append AsNoTracking() to your Entity Framework queries. EF Core skips change tracking and snapshot creation