One thing, why does 'ps aux' take about 5 seconds to complete, while
   on FreeBSD it takes roughly 0.4 seconds?

The NetBSD version is going to reread things from swap each time,
whereas the FreeBSD version will cause other things to be rotated out
of the cache the first couple of times.  It's a trade off of whether
you want to allow ps(1) to be slow, or have it affect the performance
of other programs, and whether or not you want procfs to be required.

   And, while printing the process entries, there is
   heavy hard disk i/o, most likely this that causes the slow printing.

That's when it reads from the swap space -- to get the argument lists.