My professional answer is: it depends. For generic software that doesn't use specific hardware implementations, the results aren't quite as intuitive since post-JIT processing will be at least as fast as C++ (i.e. after the same cycle has run a few times).
For hardware-based operations (OpenGL, OpenCV, custom hardware interfaces like radios) C++ is typically faster latency-wise, yet processing wise it's the same as Java. This is only due to the JNI code layer, and is implementation dependent (i.e. most of the time it isn't a noticeable difference unless you're shoving MB's of data through the code per second).
Bad programming practices will cause more performance issues on an FRC bot than the language choice.
Some articles:
http://vanillajava.blogspot.com/2011...-for-high.html
http://vanillajava.blogspot.com/2012...revisited.html
http://vanillajava.blogspot.com/2012...s-in-java.html
http://vanillajava.blogspot.com/2011...llions-of.html
And the article to make contraversy:
http://vanillajava.blogspot.com/2011...er-than-c.html