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For my app CSV Data grapher used to be all in Java. Then I started looking in optimizing it's performance all around, I did object pools, and various other tricks you can think of. It did improve in speed for rendering/loading. But not compared to the speed improvements of just doing all the parsing/loading, and reorganizing the data in C++. The rendering is still in Java but that wasn't the bottle neck. Now my app can whip through and render more than 10K data points and still be smooth when zooming/panning.
It pleases me that recently using the NDK is becoming increasingly more acceptable in this community, it used to be a taboo here.