SMP on SMP-capable system with one processor

[adp]

We have several Dell-based systems that are dual-processor capable, but have
only one processor. The FreeBSD 4.9 kernels for each system is compiled with
SMP support, even though there is only one processor on each system right
now.

Would this actually reduce performance on a single processor system? I know
that SMP kernels have to worry about special locking, and may be doing
unnecessary work.