No need to wonder, here are some benchmarks: https://www.phoronix.com/review/bsd-linux-eo2021

OpenBSD fares far worse in performance than Linux and its BSD cousins because they actively avoid supporting CPU, memory, and other features that increase performance via some shared resource because these are often vectors for information leakage and attack.

Everyone else has taken the approach of utilizing these features and getting the performance, but mitigating attacks (or class of attacks) as they are are discovered. Which is not a worse approach per se, especially if your project places a higher value on raw performance (or performance per dollar spent) and you can strengthen security at other layers.