> I suspect the ppc->x86 stuff was slower because x86 just doesn't have the registers.
My understanding is that part of the reason the G4/5 was sort of able to keep up with x86 at the time was due to the heavy use of SIMD in some apps. And I doubt that Rosetta would have been able to translate that stuff into SSE (or whatever the x86 version of SIMD was at the time) on the fly.
Apple had a library of SIMD subroutines (IIRC Accelerate.framework) and Rosetta was able to use the x86 implementation when translating PPC applications that called it.
Rosetta actually did support Altivec. It didn't support G5 input at all though (but likely because that was considered pretty niche, as Apple only released a G5 iMac, a PowerMac, and an XServe, due to the out-of-control power and thermals of the PowerPC 970).
My understanding is that part of the reason the G4/5 was sort of able to keep up with x86 at the time was due to the heavy use of SIMD in some apps. And I doubt that Rosetta would have been able to translate that stuff into SSE (or whatever the x86 version of SIMD was at the time) on the fly.