Now that Intel has moved their processor line primarily to multicore, most of Apple's shipping Macs have at least 2 cores (and as many as 8). According to Apple's Leopard pages, Apple has introduced significant performance improvements into Leopard to take advantage of all these multicore processors.
The new Leopard scheduler is very efficient at allocating tasks across multiple cores and processors. So Leopard spends less time managing tasks and more time performing computations. A new multithreaded network stack speeds up networking by handling network inputs and outputs in parallel.
Apple's applications such as Mail, Address Book and Font Utility have been updated to be multicore ready:
Each of these apps breaks up processor-intensive actions into a series of more manageable steps that execute one by one on single-CPU computers and in parallel on newer, multicore systems. Cocoa uses that same technology to speed up Spotlight searches and Dictionary lookups.
Apple also introduced a new API (NSOperation) which makes it easier for programmers to take advantage of multicore processing: "You simply describe the operations in a program along with their dependencies. Cocoa takes care of the rest."
According to one unverified first hand report, the new finder has also seen improved performance:
The new finder is absolutely the best part. How many years have we wanted a cocoa finder? ... Proper multi-threaded support. ... No more beachball so far.
