Sitecore ASP.NET SDK and slow performance on non-root paths

While working on a client solution using the Sitecore ASP.NET SDK I discovered a strange performance issue there the throughput dropped 10x on all non-root paths, home page / was blazing fast, but all other pages significant slower!

The solution is using the built-in output caching in ASP.NET and when running this:

bombardier-windows-amd64.exe -n 5000 -c 50 -t 30s https://localhost:5001/

I was getting something like ~40.000 req/sec which is very nice indeed, but doing the same any other path like /about or /about/contact I was "only" getting ~4.000 req/sec!

The culprit

Could not find anything in our own implementation that was running when a page was in the output cache so I started to look into the ASP.NET SDK source code and found the offending code: https://github.com/Sitecore/ASP.NET-Core-SDK/blob/b04d2c8ba039cdcfeb419b2514b69dafd9c4e408/src/Sitecore.AspNetCore.SDK.RenderingEngine/Routing/LanguageRouteConstraint.cs#L26. The SDK is loads all known cultures from OS with CultureInfo.GetCultures(CultureTypes.AllCultures) on every request, and it does that before the output cache kicks in!

In the solution I was working on I could fix it by just removing the call to Sitecore.AspNetCore.SDK.RenderingEngine.Extensions.MapSitecoreLocalizedRoute() (source) since we are mapping the hostname to a culture and have no need to handle the culture from the path.

If this is not an option for you then you can pick one of these options:

1. Replace the LanguageRouteConstraint when a version that just uses the IOptions<RequestLocalizationOptions> that you must likely already have configured in UseRequestLocalization.
2. Wait on the issue I created is resolved: https://github.com/Sitecore/ASP.NET-Core-SDK/issues/43
3. Or even better, create a PR with a fix.

Conclusion

Don't ever call CultureInfo.GetCultures(CultureTypes.AllCultures) on each and every request, highly unlikely the OS will get new cultures installed that often 😉 and always measure performance to catch issues like this early on!