As always: use standard libraries first, profile, then write your own if the data indicate that it's necessary. To your point, the standard library probably already uses the OS primitives under the hood, which themselves do a short userspace spin-wait and then fall back to a kernel wait queue on contention. If low latency is a priority, the latter might be unacceptable.

The following is an interesting talk where the author used a custom spinlock to significantly speed up a real-time physics solver.

Dennis Gustafsson – Parallelizing the physics solver – BSC 2025 https://www.youtube.com/watch?v=Kvsvd67XUKw