about 1-2 min to shutdown or reboot
Kernel 4.8.1 SSD drive
Here is start of messages
ata 2.00: exception emask 0x0 SAct 0x0 SErr 0x0 action 0x6 frozen
ata 2.00: cmd a0/00:00:00:08:00/00:00:00:00:00:00/a0 tag 27 pio 16327 in
Get event notification 4a 01 00 00 10 00 00 00 08 00res 40/00:00:00:00/00:00:00:00/00 Emask 0x4 (timeout)
ata 2.00: status ( DRDY )
ata 2.00: revalidation failed (errno=-5)

Machine is stock Dell i5 optiplex 9020 about 1-1/2 yrs old -- case and fan are clean inside, connections are tight, disk (short) test shows OK, Windows shuts down normally

I think problem started about a month ago, maybe same time I replaced monitor or shortly after
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I installed 4.0 kernel (I had 4.8 before) and problems disappeared
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I had a slowdown with new kernel too, I noticed it using rsync backup, slowed the entire process down about 3x or so. That was with new kernels too.

By the way, you forgot to mention which kernel you're using, that's not irrelevant.

Kernel: 4.8.0-3.1-liquorix-686-pae i686 (32 bit)
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4.8 kernel i was using was towo' from siduction site -- the 4.0 kernel i'm using now is 4.0-9.dmz.1-liquorix-amd64 ... i had tried a recent liquorix kernel and that didn't help -- i think it was 4.7

My guess is that there is a driver in the recent kernels (probably for ssd drives) that doesn't work well with my system, so the compilation doesn't make much difference
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You might talk to towo to see if you can figure it out, might be worth posting a bug report to linux kernel once you figure out what data they need.
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fixed in kernel patch bugzilla.kernel.org/show_bug.cgi?id=187061
bugzilla.kernel.org/show_bug.cgi?id=151631
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