OpenBSD runs on hardware that's been obsolete for ten years. This isn't a deliberate design decision — the hardware was in popular use when OpenBSD was started, and the developers try to maintain speed and compatibility when they can. People who are running OpenBSD on an ancient VAX quickly catch changes that badly affect system performance on 486s, while people running modern Pentium 4s would probably never notice. Some of these changes are required by the advancing nature of the Internet, changes in the tools used to build OpenBSD, and added functionality in the system, but those that are the result of programming errors or misunderstandings are caught quickly.
OpenBSD leaves you every scrap of computing power possible to run your applications. In the end, people use applications and not operating systems. This means that a system with a one-gig disk and a 486 CPU can still make a solid web server once you install OpenBSD! A low-footprint operating system gives the most bang out of hardware.