This gets repeated a lot and it's generally false. In fact, most of the time they were slower, and realistically you would expect this. On a clock for clock basis, an OG Mac Pro 2.66GHz was about 10-20% slower than a Quad G5 2.5GHz running the same PowerPC software. In some benchmarks, the Quad G5 was still faster at running PowerPC software than the 3.0GHz (see https://barefeats.com/quad06.html ). When the Core Duo had to pay the Rosetta tax, even upper-spec G4s could get past it (https://barefeats.com/rosetta.html) and it stood no chance against the G5.
Where I think this misconception comes from is that on native apps (and Universal apps have native code), these first Intel Macs cleaned the floor with the G4 and most of the time nudged past even the mighty Quad, and by the second generation it wasn't a contest anymore. But for the existing PowerPC software that was available during the early part of the Intel transition, Power Macs still ran PowerPC software best overall. It wasn't Rosetta that made the Intel Macs powerful, it was just bridging things long enough to buy time for Universal apps to emerge. Rosetta/QuickTransit was marvelous technology, but it wasn't that marvelous.