I worked on the original port of Chrome to Linux. At that time Firefox was much slower than Chrome on Linux. I think one reason was because Firefox relied more on calls into X for drawing (e.g. I believe they used xrender for compositing alpha channels), where Chrome bypassed all of that (just did all its work in a pixel buffer, which is also why we had so many problems with system fonts and system widgets etc.). Unfortunately especially on a platform like Linux where graphics drivers aren't great you're often better off avoiding the system. I imagine OpenBSD is similar -- like if they're using OpenBSD's malloc instead of their faster jemalloc that likely costs a lot.

I also know Chrome also makes a million calls to gettimeofday so it is effectively relying on the Linux vdso performance.

When you are running the same software on multiple OSes side by side (like Chrome on Windows followed immediately on Linux) it sets your expectation for what is acceptable performance. (It's probably worth mentioning here that Firefox on Linux today is much better than used to be.) I appreciate that OpenBSD has a different culture and goals but I feel sorry for them that they don't get a good browser because of it.

At least for me current Firefox on Windows locks up more often than on Linux, but that's most likely due to Windows having more coarse grained locks hanging around in the subsystems than Linux does.