That's an old ASP.NET Web Forms / ASPX thing that was IIS-based. IIS would just compile .cs files into a temporary folder when first running. So the first request takes like 5s or something.

It's not the new .NET Core AOT feature, GP was building the DLLs and packaging the website locally.

Not GP but funny enough I ran into a similar problem with a team that also didn't know compilation and was just copy/pasting into a server.

When dotnet first came out, Microsoft tried to bridge the old to the new. They included the IIS compile time capability to mimic jsp (Java server pages) and asp (Active server pages) so programmers could more easily make the leap.

Most serious developers skipped this goofiness and deployed compiled and tested builds.