My application taking so much time to launching First Activity screen

The Instant Run is enabled in your Android Studio. Try to disable the function. You will not get that issue.

Why does my App start incredibly slow (10s+) at first run, showing only white screen on android 5.0?

check this In android studio --> File --> Setting --> Build --> Remove tick following option's

if you are launching application to the first time it take time to load, after that when you open it second time it not take time to open.

You are using debug_apk. try creating release sign apk once and check it take time or not.