When I created apk file and after install , It is get 20 seconds to open.
I am using ionic 3 I use code " $ ionic cordova build --release android "
You should use the the prod flag:
--prod
for production builds, but that’s only part of the story. There are a number of things that could be slowing down your application, maybe this article can help you down the right path: https://www.joshmorony.com/ionic-framework-is-fast-but-your-code-might-not-be/
Key things to focus on when considering boot time are making sure you are creating a production build, that you are hiding the splash screen appropriately, and that you are using lazy loading (in Ionic 3, that means using @IonicPage).
--aot --minifyjs --minifycss --release
gave me much faster speed on Android.