Ah ok, so it might be wise to just drop the splash screen then and just use a static one (or some css animation as you) inside the ion-app tag. Interesting, might try that.
Also you are right that on a newer Samsung phone this did not seem to be the case, and boot time is way faster. The phone I am testing on is an older HTC m7 which has this black screen showing for a long time before the white one, although it is very intermittent. Sometimes it fades directly from the splash screen to the white one showing the ion-app content I have put inside index.html and other times it fades to the black one.
The first line in my console log when inspecting my mobile device through Chrome is this also:
"deviceready has not fired after 5 seconds."