How to Systematically Tune ASP.NET Core APIs for High-Traffic Production Load

ASP.NET Core APIs that perform well in staging often degrade under real production traffic due to accumulated small inefficiencies rather than a single bug. The most common culprit is sync-over-async code, where blocking calls like .Result or .Wait() starve the thread pool and spike latency even when CPU usage appears normal. Developers are advised to propagate async/await throughout the entire call stack, including middleware and background services, and to pass CancellationTokens so work halts immediately when a client disconnects. Output caching can dramatically reduce backend load for endpoints serving repeated data, such as product catalogs, with tag-based invalidation keeping responses fresh without sacrificing cache hit rates. The broader tuning strategy covers async correctness, caching, data access, serialization, middleware ordering, and diagnostic tooling to guide where optimization effort is best spent.