Wondering if displaying map in the app has limit for free api key ?
Also my screen having MapView loads slow for the first time(little hang), works fine after that. Any pre caching technique I should use here ?
Using the Google Maps API in native iOS and Android is free I believe but you should check the pricing in the link you posted (Platform Pricing & API Costs - Google Maps Platform) and policies here (Google Maps Platform Terms Of Service | Google Cloud).
Often times heavy native components take time to load because a lot of code and data is being loaded into RAM. You could try rendering an invisible, tiny map to preload it. Another approach is to show a loading indicator on top of where the map will go. The tradeoffs between UX and battery life and speed are up to you.