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Despite the recent development of multiple processors support in the OpenBSD, the kernel still operates as if were running on a single processor system. On a SMP system only one processor is able to run the kernel at any point in time, a semantic which is enforced by a Big Giant Lock. The Big Giant Lock (BGL) works like a token. If the kernel is being run under one CPU then it has the BGL and thus the kernel can _not_ be run on a second CPU. The network stack and thus pf and pfsync run in the kernel and so under the Big Giant Lock.
If you have access to a multi core machine and are expecting to use programs that will take advantage of the cores then the multi core board is a good choice. PF is _not_ a multi core program so it will not benefit from multi core kernel.