First, you can change this sysctls:

#vm.anonmin=70
#vm.anonmax=95
#vm.execmin=2
#vm.execmax=4
#vm.filemin=0
#vm.filemax=1

Default options are good, if you have 16-32MB of RAM, not 512. Or you can use
option PDPOLICY_CLOCKPRO to your kernel config (I don't know if it is in
NetBSD <=4, I'm using current) - then it isn't needed to change that sysctls.
It is good to add options BUFQ_READPRIO and BUFQ_PRIOCSCAN. Then you can use
"dkctl wd0 strategy readprio". You can try to change to priocscan and see, if
it's better. Available strategies you can see using 'sysctl
kern.bufq.strategies'. It helps me a lot.