I've noticed a trend where navigation throttles observe WillStartRequest, and post a task to the IO thread to do some async work.

This IO thread work blocks navigation!!! I.e. since the IO thread is FIFO this work will occur before NavigationURLLoaderImplCore::Start.

In fact, I think  issue 779914  might be in this critical path.

I firmly believe adding this metric will prevent PLT regressions moving forward, as it will be extremely low noise, and 100% in the critical path of loading (if restricted to main frames).