FreeBSD has a bit more of a lax attitude historically to security[0] and seems to prefer being reasonably performant and "easy to use" (this is subjective, but they care about supporting packages outside of base very much, and bundle non-FreeBSD produced packages as part of their base).

OpenBSD on the other hand is perfectly happy to leave oodles of performance on the table for security. They were the first OS to completely drop Hyperthreading support for example, years before spectre/meltdown.

So with these things in mind, FreeBSD is a lot more performant.

[0]: https://vez.mrsk.me/freebsd-defaults