I'd go exactly the opposite direction: If the browser is the only important thing, surely it's best to have as little overhead to running that browser - every byte of memory wasted on something else is a byte the browser could have used (and it needs enough of them!), every running process uses time and battery that the browser could have taken, every additional package/program/library is security risk on top of the secure(ish) sandbox where your real (web)apps are running.
One more reason to make it minimalist. The built in security already makes OpenBSD slow, so installing anything unnecessary will slow it down even more.
> The built in security already makes OpenBSD slow,
OpenBSD doesn't really have any security stuff that impacts performance. They're claims to security are primarily based on reasonable default configs and auditing older code.
