How can I stop this indexing or whatever this startup process? What is it for? Why is it doing it? Can I defer the loading of such huge process later on when I most need it?
You can't stop or defer this process. The process is required for the IDE to build a model of your code, which is required for all features that make a difference between the IDE and a text editor (code navigation, code completion, error highlighting, usages search, refactoring and so on).
If you do not need any of those features, you can avoid this process altogether by using a text editor such as Sublime Text instead of Android Studio.