Thank you for the feedback. Performance testing is not yet in our focus for Test Studio for APIs for the near future. Similar functionality is available in Test Studio Ultimate as its Performance Testing feature (you can see some more info about it here: http://www.telerik.com/teststudio/performance-testing and here: http://docs.telerik.com/teststudio/features/testing-types/performance-testing/details-view). This feature is mainly focused on web tests though so I presume it will not exactly fit your case, but I would suggest you to take a look at it and check if it would be useful for you. Any additional feedback is appreciated.
As for Test Studio for APIs in particular, we will still consider the option to implement at some extend similar statistics. For now, since this feature is currently available in Fiddler, you could use Fiddler for traffic capturing and performance analysis and finally import the captured requests as SAZ file into Test Studio for APIs for the means of importing them as tests. (Test Studio for APIs uses Fiddler Core for traffic capturing so the functionality is very similar across the two applications with Fiddler offering richer UI).