The biggest area of improvement in NetBSD 5.0 is symmetric multiprocessing (SMP). "NetBSD 5.0 features greatly improved performance and scalability on modern multiprocessor and multi-core systems. Multi-threaded applications can now efficiently make use of more than one CPU or core, and system performance is much better under I/O and network load, benefiting, for example, server, scientific, and software development workloads."
These improvements were made possible by rewriting or auditing just about every core aspect of the system. Instead of paraphrasing these changes and making mistakes, let's just look at what the release notes have to say:
There were also a huge number of other changes, such as the "jemalloc" memory allocator, Xorg instead of XFree86 in some ports, the Power Management Framework, ACPI suspend/resume on many laptops, and much, much more.