Mobile App taking a long time to load only on Android

I have a mobile app I am working on using AIR19 and Intellij.

On iOS it performs fine, taking around 4 seconds to load.

but on Android (version 5.1) it takes 35 seconds. Which is killer for my app.

I have added  timing points to time how long it takes to load, and what I find is the app is ready to go in about 4 seconds (reported via dump of startup time log to bugsnag after startup).

Then something is leaving it with a black screen for the next 30 second.

This is true for both the debug and release version.

Yesterday by accident I reduced the time down to 6 seconds.

But then it changed back to 35 second by apparent magic!

Any clues as to how to determine what is happening?