Having IPSEC compiled into the kernel imposes a non-trivial performance penalty on multi-threaded workloads due to IPSEC refcounting.
In my benchmarks of multi-threaded UDP transmit (connected sockets), I've seen a roughly 20% performance penalty when the IPSEC option is included in the kernel (16.8Mpps vs 13.8Mpps with 32 senders on a 14 core / 28 HTT Xeon 2697v3)). This is largely due to key_addref() incrementing and decrementing an atomic reference count on the default policy. This cause all CPUs to stall on the same cacheline, as it bounces between different CPUs.
Given that relatively few users use ipsec, and that it can be loaded as a module, it seems reasonable to ask those users to load the ipsec module so as to avoid imposing this penalty on the GENERIC kernel. Its my hope that this will make FreeBSD look better in "out of the box" benchmark comparisons with other operating systems.