Hi,
Technically 'yes' we can use Selenium test in a Load test (at least in LoadTest from MS Visual Studio).
But if the test scripts have lot of wait (or sleep) statements to perform something on the page, then these will not give a picture of how a test case is performing. So, usually UI Tests wont be included in performance testing.
If the test code does not have any wait statements, you can try executing them in Non-UI mode, like using a PhantomJS or a HTMLUnitDriver (headless browsers). These would be a better fit for load testing.
Other options that I know of is to use "Web Performance testing" in load test. This is also another way of testing the UI responsiveness
w.r.to load.
If you want to know how to include Selenium test in load test, below is one way (C# and Visual Studio)
1. Write tests using MS Test framework
2. Use Selenium libraries to automate
3. Use LoadTest wizard to create a load test. Include the test method created above.
Regards,
Praveen