I have upgraded my Macbook to Snow Leopard this weekend and indeed experienced better performance. I have bumped into an interesting article which explains why : (follow link )
To make short story shorter Mac maps 32bit processes to two 4GB address spaces one for user mode and one for kernel mode. As a result any kernel mode system call will require a TLB flush.
Windows by limiting user address space to 2GB and kernel address space to 2GB do not suffer from this problem (but suffer from much lower memory limit). Snow leopard is full 64 bit which solves this issue. User mode and kernel mode address space share the same TLB