App starting much faster without network

While testing some offline scenarios, I noticed something weird today:
When the smartphone has network, starting the app takes several seconds. But switching to airplane mode the app starts within <1s. This is reproducable.

Did anybody else encounter this?
So my app is actually fully offline working, network is only needed for ads and tracking purposes. I have no server for the app so the app itself won’t load any data. The ads should be loaded after the app is loaded completely actually, so this should also not block it.

I somehow think this might be due to some cordova plugin.
This is my list of plugins:

 <plugin name="cordova-plugin-x-socialsharing" spec="^5.3.2" />
    <plugin name="ionic-plugin-keyboard" spec="^2.2.1" />
    <plugin name="cordova-plugin-ionic-webview" spec="^1.1.19" />
    <plugin name="cordova-plugin-market" spec="^1.2.0" />
    <plugin name="cordova-custom-config" spec="^5.0.2" />
    <plugin name="cordova-plugin-device" spec="^2.0.2" />
    <plugin name="cordova-plugin-splashscreen" spec="^5.0.2" />
    <plugin name="cordova-plugin-statusbar" spec="^2.4.2" />
    <plugin name="cordova-plugin-whitelist" spec="^1.3.3" />
    <plugin name="cordova-sqlite-storage" spec="^2.3.0" />
    <plugin name="cordova-plugin-inapppurchase" spec="^1.2.0" />
    <plugin name="cordova-plugin-admobpro" spec="^2.31.4" />
    <plugin name="cordova-plugin-inappbrowser" spec="^3.0.0" />
    <plugin name="cordova-plugin-firebase" spec="^1.0.4" />
    <plugin name="cordova-plugin-local-notification" spec="^0.9.0-beta.2" />

Maybe somebody else can also test this and share his results with his installed plugins?
I think mainly admobpro or firebase could cause this.

I remember I have seen github issues in admobpro some time ago where it was claimed the app phones home to check for licencing stuff. Maybe this is slowing it down so much? But the github page of admobpro has issues disabled since several months… (not very trustworthy for a plugin providing money-related-functionality?)