"fast enough" - you are running applications on a mobile phone that question relative to what you expect it to do.
Also java's performance has been increased through the years. But also run you are just coding in java. Google has created a tool that take java byte code and converts it to .dex (Dalvik Executables). Dalvik the VM implementation that java used is optimized for this platform. Which mean you can program in a high level language like java which provides many helper classes which means you will be more productive.
They do have a NDK but I believe you have to still have a java front end and have your java code call the NDK code when necessary. You pretty much only want to write the native code when you really need to optimization. Alot of java classes like java nio and the opengl classes are just wrappers to low lever calls that use hardware resources directly.
I probably over answered you question a little.
