an ASP.NET performance bottleneck mystery

Question

A classic ASP.NET app - AppSrv + MS SQL DB. Both servers are heavy-lifters 8 cores, 20 GB of RAM. When load testing, the throughput goes somewhere to 400 VirtualUsers (according

Answer 1

Does the host utilize App Pool?

Did you try increase the number to 5 to 10 in

An Application Pool -> Performance -> 
Web Garden -> Max Number of worker processes