When users hit your site for the first time (especially after a period of inactivity or a deployment), you may notice a noticeable lag before the page loads. This “cold start” phenomenon is common in .NET web apps. In this article, we’ll explore why that happens and share actionable techniques to optimize your .NET site so that even the first request is snappy.
Why the First Request Is Slow
Before we jump into fixes, it’s helpful to understand what’s happening under the hood.
- Cold start / process spin-up: If the app is idle or has been recycled, the web server (IIS, Kestrel, etc.) needs to spin up the process, allocate memory, load assemblies, JIT-compile methods, and initialize services.
- JIT compilation & dependency resolution: Methods and dependencies that haven’t been compiled or warmed up must be just-in-time compiled before they can execute.
- Initial application startup logic: Your Startup or Program logic may run database queries, dependency injections, configuration loads, or other heavy initialization.
- Database or external service initialization: Opening initial database connections, fetching configuration from remote systems, or warming external services can also add delay.
- HTTP / network or DNS overheads: Domain name resolution, TLS handshake, certificate checks, and initial DNS lookups can contribute.
- Application pool recycling / idle timeout: In IIS, if the application pool is recycled or idle for some time, the app has to restart for the next request.
Because a lot of this work is done only once, subsequent requests tend to be much faster.
Best Practices & Techniques to Optimize First-Request Performance
Here are strategies you can apply (some immediately, others more effort) to reduce cold-start latency and ensure your first visitor doesn’t wait too long.
1. Enable Application/Pool Preload & “Always Running” Mode
- IIS Auto-Start / AlwaysRunning: For ASP.NET (pre-Core) hosted on IIS, you can configure the application pool with startMode="AlwaysRunning" and enable service auto-start on the site. This ensures the app starts up without waiting for a user request.
- IIS PreloadEnabled: Set preloadEnabled="true" on your site in the IIS settings so that IIS will “warm” the site at startup.
- ASP.NET Core in IIS Hosting: Use the IISHostingStartup or an initialization middleware to cause work during startup rather than lazily in the first request.
- Warm-up scripts: Run a “warm-up” HTTP request to key endpoints immediately after deployment (or periodically) to keep the app alive.
2. Reduce or Defer Heavy Startup Work
- Lazy initialization: Delay heavy initialization (e.g. expensive database queries, third-party service calls, or large cache loads) until they are actually needed, rather than during startup.
- Pre-build or pre-warm critical services: For services you must have ready, consider constructing singletons or warming them during startup rather than on-demand.
- Split initialization into background tasks: If some work isn't required to satisfy the first request, run it in the background after the response is sent.
3. Optimize Dependency Injection / Service Registration
- Avoid overly complex DI graph resolution during startup.
- If you have many singleton dependencies or large object graphs, ensure that only the minimal set is resolved when the app starts.
- Use IHostedService or startup tasks carefully to avoid blocking the request pipeline.
4. Minimize JIT & Precompile
- Use ReadyToRun / Native AOT (in newer .NET versions) or publish with tiered compilation so that much of the code is precompiled and less JIT work is needed at runtime.
- Precompile views (for ASP.NET MVC or Razor pages) so they don't need to be compiled on first request.
- Reduce dynamic code generation or reflection-heavy patterns that cause runtime compilation.
5. Use Caching Wisely
- Memory caching / distributed caching: Cache data or computed data so you don’t have to fetch or compute it on each request.
- Response caching: Use ResponseCache or caching middleware to cache full or partial responses for reuse.
- Output caching / fragment caching: Cache parts of page rendering that are static or semi-static.
- Preload key cache entries (e.g. lookup tables) during startup if they're needed early.
6. Asynchronous, Nonblocking Code
- Avoid blocking calls (e.g. .Result, .Wait(), synchronous I/O) in your startup or request pipeline.
- Use async APIs throughout (database calls, network calls, file I/O) so threads aren’t blocked and can serve other requests.
- Avoid locks or other synchronous constructs in common code paths.
7. Optimize Database / External Call Overhead
- Use connection pooling (usually enabled by default) so initial database connections are fast once warmed.
- Ensure your queries are efficient, optimized, and indexed so that retrieving initial data is fast.
- Use AsNoTracking() for read-only queries if using Entity Framework.
- Avoid many small chattiness calls; batch work where possible.
- Consider caching external API results where practical.
8. Optimize Static Files, Compression & Network Layers
- Serve static assets (CSS, JS, images) with aggressive caching headers and CDN support so those don’t delay subsequent requests.
- Enable response compression (Gzip, Brotli) to reduce payload size.
- Use HTTP/2 or HTTP/3 if supported, to reduce connection overhead.
- Use connection keep-alive where possible.
9. Monitor, Profile & Instrument
- Use profiling / tracing tools (e.g. Application Insights, PerfView, MiniProfiler) to identify exactly which parts of initialization or first requests are slow.
- Log timings for your startup, DI resolution, first-controller handling, database queries, etc.
- Measure before and after for each change to ensure your optimization is effective.
10. Keep Application Alive
- Use a keep-alive or health-check job (e.g. ping a lightweight endpoint every few minutes) to prevent the application from going idle or being recycled.
- In serverless or containerized environments, ensure your deployment doesn’t scale to zero or sleep.
Checklist for Your App’s First-Request Optimization
- Configure your hosting (IIS / Kestrel) for preload / always-on behavior.
- Publish with precompilation (views, ReadyToRun, AOT) if available.
- Reduce heavy startup logic; defer or background initialization.
- Warm caches or data that will be needed early.
- Use async and non-blocking code everywhere possible.
- Enable response caching, compression, and CDN or static file caching.
- Monitor and profile to find which steps are slow.
- Use keep-alive pings or scheduled calls to prevent idle timeouts.
Final Thoughts
A slow first request is almost always due to the combination of process cold-start, JIT compilation, DI / initialization overhead, and external dependencies. But it's also a solvable problem. By applying a combination of warm-up strategies, reducing unnecessary startup work, precompiling, caching wisely, and instrumenting your app, you can dramatically reduce the latency for your users—even for their very first visit.