JDK 5 -> JRE 6 perfs

[broumbroum]

HI! I don't think this was asked before, here's an issue I want to understand :
I'm having trouble getting reasonable perfs on my game since I changed the JDK 6 for JDK 5 (Windows and Linux), because of the sources  level that I want to be useable on Mac OS JRE5. Now that I compile with JDK 5, it looks like if the Java 6 JRE which I'm using to run tests is running in "interpreted mode", such as the perfs are being downgraded.
Is this "interpreted mode" a realistic issue ?