the test run registration (e.g. for Web API testing) is not supported for load testing scenario. For load testing, it makes more sense to have a look at the overall results of the load testing, and not to compare directly each requests.
For load testing, have a look at this page in the documentation: https://www.dynatrace.com/support/doc/appmon/integ...
Also Andi Grabner has a great video on our youtube channel: https://www.youtube.com/watch?v=26nVM_JgStw
hope that helps,