I think that I know the reason for this.
A lot of things happen while the CMS is working, it is not just serving the html content to the browser. Some providers are being initialized, security checks are being performed, queries to the database are being executed and so on.
You can optimize all of this by enabling the cache. In local tests, the CPU was not going over 20% with caching enabled and 250 concurrent users, since a lot processing is just skipped. The caching for the pages could be found under Page -> More option.
You should also disable the Sitefinity debugging through the web.config file - <compilation debug="false">. This is really important thing to do when you go live, because in the other case, debugging symbols are being load which is a great overhead.
Please let me know if these suggestions were helpful for you.
the Telerik team