I did some experiments on my local machine and found out that IRIS seems to associate each request (eg: CSP request) to a dedicated process named IrisDB.exe
This process has 3 threads, and one of them seems to be the main working thread (that will the handle request).
If I spawn many concurrent requests, the number of IrisDB processes on my local machine will grow (to make sure each request can be handled in parallel).
It seems there is a limit : it will create no more than 64 processes to handle requests (eg: even if 100 concurrent requests are send to IRIS).