There seams to be a maximum parallel tile requests when using a wmts service in ArcGIS pro, similar to how web browsers throttle the number of requests to a single domain.
How to reproduce:
add a wmts service to your map in ArcGIS pro, zoom in and look at the web request logs
What I expect:
In arcgis 9.x and 10.x the server receives all the tile requests at once and is able to fulfill all of them simultaneously
What actually happens:
I see some requests, and then after they finish I see more coming in making the the layer take much longer to load
Idea:
remove the limit and make sure ArcGIS Pro wmts performance isn't regressed compared to 10.x