It might sound banal but I actually help a guy few days ago. I got a phone call that ASP.NET application that he develop consuming 50% of CPU with minimum server pressure and climb to 100% CPU when request start to come rapidly.
That guy develop simple ASP.NET application, no multi-threading or other complexly just Select/Insert/Update application. So it looks very strange to me that CPU goes so high.
Well the reason was that he use Tracing as logger to write down what is going on in his application without being aware that ASP.NET write is own stuff into trace data. When he stop tracing, CPU just return to normal values. We end up by using Nspring to write a-synchronous into a log file, ending up with better RPS.