According to the article from Ian Douglas:
The stock Nexus One with Android 2.1 averages in the high 6.9 to low 7.0 range for MFLOPS, but the Nexus One running the Debug release of Android 2.2 (aka FroYo, “frozen yogurt”) can apparently achieve a score of up to 40 MFLOPS. My own testing confirms it at a high-36 to mid-37 range:
According to AndroidCentral, that equates into a roughly 450% increase. What exactly is a JIT compiler and what does this all mean? From what I have gathered so far on AndroidPolice, a JIT compiler helps the computer improve performance for programs that are not "compiled to native code, which Flash and some video codecs/drivers seem to be."
A comment on AndroidandMe's article covering the story gives some insight into the real-world benefits for typical users:
A JIT compiler doesn’t require specific tuning. Things will just run better and most definitely real time apps & games in Java (yeah!). Anything that has a near real time loop that executes over and over (a game loop) will benefit extremely well from this and not require any tuning.
Google IO should bring some light onto this whole situation.
Image from Ian Douglas