NSURLSessionTaskPriority seems to be ignored?

The priority does not influence the priority on your app's side. It is a hint for the host to response to several requests in a priority. The host can completely ignore that.

To provide hints to a host on how to prioritize URL session tasks from your app, specify a priority for each task. Specifying a priority provides only a hint and does not guarantee performance. […]

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You can specify or change a task’s priority at any time, but not all networking protocols respond to changes after a task has started. There is no API to let you determine the effective priority for a task from a host’s perspective.