One theory I have about bug 1682319 is that we might be seeing a "double deprioritization" of content processes:

Suppose Fenix is active with an active tab of web content. That tab's content process is considered by the ProcessPriorityManager to be high priority, so we bind its service with the Context.BIND_IMPORTANT flag.

Now suppose I switch to a different app. The entire Fenix app will be de-prioritized by Android by virtue of the fact that it is backgrounded. However, the ProcessPriorityManager also receives notice that the app has backgrounded, and decides to dispatch a priority drop for the active tab to GeckoProcessManager. We drop the Context.BIND_IMPORTANT flag from the active tab's service bindings, thus deprioritizing its content process over and above whatever was automatically done when the Fenix app was backgrounded.

My hypothesis is that the ProcessPriorityManager should probably not deprioritize the active tab's process(es) when the app is backgrounded on Android.

Duplicate of fixed bug 1753700