Most .NET Core APIs don’t fail because of bad code.

They fail because of bad assumptions.

After working with ASP.NET Core in real projects, this is what I’ve learned the hard way 👇

🔹 Async/await doesn’t mean scalable
If your async code blocks threads (.Result, .Wait()), you’ve already lost under load.

🔹 EF Core is not slow misuse is
Tracking everything, lazy loading blindly, and ignoring generated SQL is what hurts performance.

🔹 Stateless APIs are not optional anymore
If your API stores user state in memory, horizontal scaling will eventually break you.

🔹 Caching is a design decision, not a fix
Add caching without a clear invalidation strategy, and you’ll trade speed for bugs.

🔹 Most “slow APIs” are blocked, not busy
The database is waiting on locks, not executing slow queries.

🔹 Retries without idempotency are dangerous
They don’t improve reliability they multiply failures.

🔹 Rate limiting is about survival
Not speed. Not optimization. Survival.

🔹 Security is not just authentication
Authorization failures are the most expensive bugs in production.

🧠 Big takeaway
ASP.NET Core is powerful, but it doesn’t protect you from poor system design.

Frameworks don’t scale systems.
Engineering decisions do.

Sharing this for anyone building or interviewing for real-world .NET backend roles.