To summarize what's in pieces here, the G3 is an updates 603 core. The 604 is capable of dispatching more instructions per clock than the 603 or G3 and the FPU is definitely faster. The reason that the G3 is generally faster is that the G3 has an L2 cache controller built-in and therefore almost always has fast L2 cache (133 MHz to 200 MHz as opposed to 50 MHz for the 604(e) and 100 MHz for the 604ev) and almost always used a faster SDRAM bus (66 MHz SDRAM as opposed to 50 MHz (typical) FPM).
If you were to put a 604e onto the same speed memory bus with the same amount of L2 cache also running at the same speed as a G3, it'd be faster than the G3 at everything.

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