How does Visual Studio process the App_Code folder specially?

How does Visual Studio process the App_Code folder specially?

How does Visual Studio process the App_Code folder when a change is made
or detected in it? Not IIS or ASP.NET.
I want to gain a better understanding of why Visual Studio freezes for
long periods of time whenever I save a code file inside a large App_Code
folder of a website project and not elsewhere. Alternatively, I could ask:
why does Visual Studio not exhibit these freezes when processing a code
file inside a class library that is equally large?
Ideally I would like to see official documentation cited from Microsoft of
the issue at hand of processing the App_Code folder in Visual Studio and
what happens that differs in Visual Studio compared to e.g. class library
processing.