Right now, from what we've heard, the real-world performance of the Xenon
CPU is about twice that of the 733MHz processor in the first Xbox.
Considering that this CPU is supposed to power the Xbox 360 for the next 4 -
5 years, it's nothing short of disappointing. To put it in perspective,
floating point multiplies are apparently 1/3 as fast on Xenon as on a
Pentium 4.
The reason for the poor performance? The very narrow 2-issue in-order core
also happens to be very deeply pipelined, apparently with a branch predictor
that's not the best in the business. In the end, you get what you pay for,
and with such a small core, it's no surprise that performance isn't anywhere
near the Athlon 64 or Pentium 4 class.
The Cell processor doesn't get off the hook just because it only uses a
single one of these horribly slow cores; the SPE array ends up being fairly
useless in the majority of situations, making it little more than a waste of
die space.
(...)
The most ironic bit of it all is that according to developers, if either
manufacturer had decided to use an Athlon 64 or a Pentium D in their
next-gen console, they would be significantly ahead of the competition in
terms of CPU performance.