I have a Visual Studio 2015 load test that I am running against SharePoint 2013. Once the test increases to around 100 users, page response times increase dramatically. Once the load test hits 1000 users, the page response time is in excess of 100 seconds. However, if I navigate to the site there is no slowdown whatsoever (even from the workstation executing the test); also the WFE servers are consuming 20% CPU, <50% RAM, and <5% network utilization during the test. The CPU, RAM, and network utilization are also in great shape on the computer running Visual Studio 2015. I am not running this test with any agents, only from one workstation. Has anyone seen this before? Could SharePoint be throttling my requests?
Go to your Central Administration website->Manage Web Applications->Click to the right of the name of your SharePoint website->Click on the little arrow on the bottom right of the "General Settings" button on the Ribbon, and choose "Resource Throttling".
There are a number of settings here you might be interested in, but you might want to take a particular look at "HTTP Request Monitoring and Throttling".
Back in "Central Administration", you might also want to check the "Monitoring" tab, as the Health Analyzer, Health Reports, and Diagnostic logging might help you track down what is going on.