I have noticed when calling a Java Servlet (this type) or an XPage with an ExtLib REST Service custom control in it, the server responds to only 6 requests at a time.
The way I have been testing the server is to open around 30 tabs in Google Chrome with Dev Tools open to disable the cache, and get network times. I have a Chrome extension that refreshes all the tabs at once. I refresh all tabs and keep track of the tab response times.
The service I am calling returns a lot of address book entries, I typically pass a parameter of 20000 or 40000 which is the number of entries returned.
An entry being of the form: {"$21":"","@UNID":"ECABACF30AAB898085257BFD006EF2E2","$12":"","CompanyName":"","$16":"Jeffery Aaron/ROCKALL","@position":"5","$17":"Aaron , Jeffery","$18":"0.0"}