It is a performance boost, and yeah it's for Intel Macs, which puts PPC macs (some of which are just as old as the Intel Macs) out of commission on that one. At one point you could buy dual core PPC or dual core Intel Macs. Anyone who chose PPC over Intel has lost out, even though they were manufactured at the same time.
What I'm getting at is that if Snow Leopard doesn't give you anything that Leopard doesn't except performance due to intel optimizations than Leopard
is the PPC version of Snow Leopard, and it runs on some seriously old kit. I've got it on an 8-year-old Quicksilver G4 for instance, and it runs faster than Tiger before it, which runs faster than Jaguar before it.
If there'd been no massive architecture change I'm sure Snow Leopard would run on machines at least 6 years old.