netbsd-6 slow after resume from S3 sleep

[Christoph Badura <bad%bsd.de@localhost>]

I'm trying to figure out why after resuming from S3 sleep my T500/W500
thinkpads are extremely sluggish when running 6.x.

E.g.:

Script started on Sat Sep 13 18:45:28 2014
size-isnt-everything# cat ./timeit
time sh -c 'seq 1 1000 | while read s; do ( true ); done'
size-isnt-everything# ./timeit
        0.20 real         0.08 user         0.11 sys
size-isnt-everything# sysctl -w machdep.sleep_state=3
machdep.sleep_state: 0 -> 3
size-isnt-everything# ./timeit
       16.22 real         1.98 user        14.22 sys
size-isnt-everything# startx
size-isnt-everything# ./timeit
        0.20 real         0.07 user         0.12 sys
size-isnt-everything# exit

Script done on Sat Sep 13 18:46:36 2014

This happens only for amd64. It does not happen when running i386.

Starting X makes the system normally fast again.  Suspending while running
X makes the system slow again.  Exiting from X then makes the system
normally fast again.

A -current kernel does not have this problem.

Has anyone else seen this problem? Perhaps on a different system?
Does anyone remember when it went away?

NB that the acpi vbios reset sysctl has to be set to either 0 or 2 for the
T500/W500 to restore video after resume.  Neither value makes a difference
with regard to the slowness.