I've used in the past (before changes to licensing model) Visual Studio Web Performance
https://docs.microsoft.com/en-us/visualstudio/test/quickstart-create-a-load-test-project?view=vs-2019
depending on your components it can be very simple or very complicated, I remember in the past having loads of issues with Web Forms, but the rest was very easy
with the new licensing model I am not sure how this would work, you can set the same user for authentication, so it would charge just one login during your tests per day (which is fine)
but I am not sure how it would work the page views, as you can set this up to run hundreds of users at the same time, might be worth to talk to Microsoft before running any stress test
do you have any concerns in general with your Portals?
an overall comment is that the Portals is not the fastest website, there is a lot of communication back to CDS and this slows down the Portals, Microsoft is being doing great improvements with caching and other little things, so it is much better than what used to be
there was a known performance issue with web forms which Microsoft claims to be fixed also
