npfctl show hangs for a long time

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

I've a npf ruleset. It's not enormous, according to npfctl show it goes up 
to id="74".

However, running npfctl show takes around 3 minutes to run and it runs at 
100% CPU:

0.119u 186.256s 3:09.30 98.4%   0+0k 0+0io 0pf+0w

ktrace/kdump shows it hanging at this point:

 19226      1 npfctl   RET 
__fstat50 0
 19226      1 npfctl   CALL  ioctl(3,IOC_NPF_VERSION,0x7f7fff2e42fc)
 19226      1 npfctl   GIO   fd 3 read 4 bytes
       "\^V\0\0\0"
 19226      1 npfctl   RET   ioctl 0
 19226      1 npfctl   CALL  ioctl(3,IOC_NPF_SAVE,0x7f7fff2e4290)
 19226      1 npfctl   GIO   fd 3 read 24 bytes
       "\0\M-@{\M-w\^?\^?\0\0\M^_'t\0\0\0\0\0\0\0\0\0\0\0\0\0"
 19226      1 npfctl   RET   ioctl 0

I've compared this to another installation with a slightly smaller ruleset 
(goes up to id="56") which takes a more reasonable time:

0.019u 1.647s 0:01.66 99.3%     0+0k 0+0io 0pf+0w

Both on NetBSD 9.1/amd64

--
Stephen