Java is not really slow. If you app is slow, probably you are doing it the wrong way. See Android Studio? It runs well with my laptop (Celeron CPU dual-core 1.6Ghz, 6Gb RAM, HDD).
For generic algorithms (such as sort, find, ...), Java's performance is comparable to C/C++. This info is not very new.
Developing a large application is another stoty. C++ allows you to control many things, including low level code that interacts with CPU/RAM/Disk while Java abstracted those to provide a cross-platform API for you. It means that you have chance to optimize your code and you eventually find some ways to make it run faster for your targeted platform with C++. From Java's optimization perspective, you have no other choice except to customize JVM's behavior to achive significant performance boost.
There're plenty of games written in Java and I would say if your game is large but still runs fast in Java (and you're already confident with Java), do it right away, C++ takes long time to master.
A recent Java game that makes me interested.