Well, as far as performance is concerned (and not necessarily scrolling, since the slowdowns have happened to me on a PC running XP...P4 and all...when the page is littered with flash and other performance consuming web stuff), remember that Mac OS X was taken from OPENSTEP, which was released after NeXTSTEP.
OPENSTEP was made to run on Pentium CPUs. Of course, when Apple bought NeXT and started work on Rhapsody (which would eventually become OS X), they had to rewrite a lot of code to be native for PPC. Of course, initially there were two versions of Rhapsody known to users during this early development phase: the PPC version and the x86 version. However, the x86 version was said to have been dropped in favor of producing the final PPC version. Of course, now we know the truth that Jobs had the x86 version (all iterations before the name OS X) being worked on all along. Of course, there were also tips to this since Darwin (the open source UNIX portion of OS X) has been available for both hardware platforms since the development began on OS X.
From what I've read lately, the x86 version of OS X is spanking the PPC version. The reason for this is because initially OS X was optimized for x86 CPUs (when it was OPENSTEP). Because of this, it was easier to switch CPUs than it was to yield the most optimal performance as possible on the PPC side by rewriting the entire OS. For all we know, this was Jobs' plan all along since he got back to Apple: to finally bring NeXT technology using a vehicle that had brand name recognition (Apple). Remember that his laptop before he went back to Apple was a Pentium laptop running OPENSTEP. I'm sure this is what he's wanted all along.
So yeah, it kind of makes you think that the performance on the PPC side is slower, but because it was probably never intended to be used optimally on the PPC. The PPC was probably just a holdover until the x86 version of OS X was ready for prime time.
If you're talking about Linux on the PPC, it definitely performs better than OS X (AnandTech did a comparison of OS X and PPC Linux and noted that OS X took significant performance hits when put against Linux/ppc). The only problem is module support from video card manufacturers which is why desktop Linux on the PPC isn't that great. However, PPC Linux development looks to be blowing up soon enough especially with this OIN alliance on patent sharing and the recent announcement of a GPL-ed PPC hardware design from Genesi, the makers of the Pegasos PPC machines.
As for OS X, we'll have to see for ourselves just how much of a performance increase we will see on the Intel side. From what I hear, OSx86 screams compared to OS X running on the PPC. Again, I don't blame the PPC platform for this....Apple has been known to purposely cripple great CPU designs in favor of their own agenda (Mac LC and LCII, Mac IIvx, PowerPC 603/603e based PowerMacs like the 52xx/62xx series, and now PPCs running OS X).