Iris Xe performance is much faster on average than with the older UHD Graphics 620. Unfortunately, GPU clock rates cycle heavily over time for unsteady frame rates and performance.
https://www.notebookcheck.net/Thinking-about-buying-the-Dell-Latitude-15-9520-2-in-1-You-may-want-to-read-this-first.581201.0.html
If you still got this laptop, you can try removing Intel DPTF and try tests again
On older latitudes there were similar issues when GPU was loaded, removing DPTF helped to stabilize freq