Maybe this is just a bad example, but I was digging through the timing wheel code of the kernels of all three main BSDs at one point. FreeBSD and NetBSD had nearly the same code. OpenBSD's was rather simpler and did not have a lot of the locking code that the other two had. My guess is that OpenBSD has either a giant lock or its kernel only runs on one core.
Most of the OpenBSD kernel is under a big lock but they are adding more fine grained locking. OpenBSD didn't add kernel threads until 2012. A bit late in comparison to freebsd and linux but it wasn't a priority to them. It also allowed them to benefit from the lessons everybody else learned the hard way.
