I was profiling the startup performance of my app, and found that initializing Realm took around 100ms, and half of that time was spent on a single reflection call

When calling Realm.init(), the following line is executed:
https://github.com/realm/realm-java/blob/afc79e568d3f710fcdf99d30bf67b33c8134f6c6/realm/realm-library/src/main/java/io/realm/Realm.java#L270

Because this configuration is created empty, build() is forced to call isRxJavaAvailable()
https://github.com/realm/realm-java/blob/afc79e568d3f710fcdf99d30bf67b33c8134f6c6/realm/realm-library/src/main/java/io/realm/RealmConfiguration.java#L812-L814
https://github.com/realm/realm-java/blob/afc79e568d3f710fcdf99d30bf67b33c8134f6c6/realm/realm-library/src/main/java/io/realm/RealmConfiguration.java#L422-L432

We can avoid this reflection call when creating our own RealmConfiguration by providing it with an RxObservableFactory, but we don't have access to the one that's created inside of Realm.init(), forcing us to waste 50ms at app startup

Proposed solution

If we could provide our own RealmConfiguration to be used for Realm.init(), then this should help improve startup performance for apps who know that they will be using RxJava