In my test, I sat up 10 tiled raster layers running on the same server. I am fully aware this setup is quite far from optimal, but unfortunately we have to live with this solution until we get access to the raw data to produce vector tiles.
My unit test adds 100 requests to the default url session, simulating a large iPad loading 10 map layers:
* Service 1: 10 random tiles
* Service 2: 10 random tiles
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* Service 10: 10 random tiles
Test 1: Using default url session, one port. Sever log: Max 5 concurrent requests processing at the server side.
Test 2: Using default url session, one port per layer: 5 concurrent requests processing per port (50 concurrent requests active on server, as expected)
Test 3: Using the runtime, multiple ports, through the runtime: Max 5 concurrent requests, causing the map to first load one layer (with only 5 active concurrent requests).
In our case, making it possible to set the max num concurrent requests per host requests would be great to drastically improve user experience.