When a .NET API works perfectly in staging and then falls over under real production load, the root cause is almost never a single catastrophic bug. It is usually a collection of small inefficiencies — a blocking call here, an untracked entity graph there, a misconfigured middleware pipeline — that only become visible once concurrent request volume climbs into the thousands per minute. ASP.NET Core performance tuning is the practice of finding and removing those inefficiencies systematically, rather than reacting to outages after they happen. This post walks through the areas that matter most for high-traffic APIs: async correctness, caching, data access, serialization, middleware ordering, scaling, and the diagnostic tools that tell you where to actually spend your time.
Async/Await Done Correctly
The single most common performance killer in production ASP.NET Core applications is sync-over-async code — calling .Result or .Wait() on a task instead of awaiting it. This pattern blocks a thread pool thread while it waits for an I/O-bound operation to complete, and under load it triggers thread pool starvation: the pool has to grow slowly, new requests queue up waiting for a free thread, and latency spikes across the entire application even though CPU utilization looks low. The fix is straightforward in principle — await all the way up the call stack — but it requires discipline across an entire codebase, including in places developers do not expect, like static constructors, custom middleware, and background service loops.
