ASP.NET Core applications often run fast locally but slow in production due to environment differences, resource constraints, database query inefficiencies, async/await misuse, excessive logging, cold starts, improper caching, and network latency. The guide covers common bottlenecks like EF Core N+1 queries, thread pool starvation from blocking calls, logging overhead, in-memory cache limitations in scaled environments, and HttpClient socket exhaustion. It provides code examples showing problematic patterns versus optimized alternatives, emphasizing measurement-driven optimization: timing requests and database queries, checking thread pool health, reducing allocations, and implementing distributed caching before scaling infrastructure.