I tried OpenBSD recently and found it behaves very differently from other OS. The same code works on Linux/FreeBSD/Windows but has poor multi thread performance on OpenBSD, async socket stopped working after sending at high speed for few seconds. I am not saying there is anything wrong in OpenBSD, it is just different.

Is the code you ran on your OpenBSD available (e.g. on GitHub) for others to test? Curious what async issue you faced, did you report it? Or ask for help addressing?

OpenBSD uses a Giant Lock model (simpler code) instead of the fine grained locking mechanism in Linux. And Linux has a some quirks and hacks to improve performance (instead of doing the slow, but correct thing). One example is the USB Gadget thing.