I have ASP.NET 4.6 application that is designed as Web API. It has one long running operation that takes about 60 seconds, but this operation isn't heavily loaded, let's imaging that like
The root cause is in IIS limit of concurrent requests execution for Windows 10 Enterprise: there are maximum 10 requests can be executed simultaneously. Actually I had 46 threads for the entire process and only 10 threads were from the thread pool. When I had tried to run my application on Windows Server 2012, I didn't experience any issues, so 400 threads were created in the thread pool during few seconds.