Setting it up initially, I raided the 4 drives together with two
  partitions on each of the components: a small one for RAID-1 to load
  the kernel, and a large one for RAID-5.  Unfortunately, the RAID-5 had
  horrible performance: 2-3MB/sec sometimes and never higher than about
  12MB/sec.

I am not 100% clear on this, but I have the impression that RAID-5
requires read-modify-write and that in the event of system crash or
power loss you can get corruption, and thus the good hardware
controllers have a) battery backed RAM and b) code that won't crash.  (I
don't mean to malign the raidframe code - but because it's in-kernel if
the kernel crashes for any reason - not unheard of - then pending raid
writes may not happen.)

Because of this I've always just bought two big disks and done RAID-1.

Perhaps Greg Oster will chime in, and it would be a good addition to the
Guide to discuss the wisdom of using RAID-5.
http://www.netbsd.org/docs/guide/en/chap-rf.html