This is a known issue that is currently being worked on I think.
I’ve found the --prod --release flags when building the application speed up the white screen.
Then for extending the splashscreen so your users don’t see the white screen you can alter the splash screen timer like so…
<preference name="SplashScreenDelay" value="10000"/>
This will let the splashscreen show so long as the white screen is there. Of course your users will still see it if the app sleeps and restarts