Comment # 37 on bug 951548 from Takashi Iwai

Well, there aren't so many things that can make suspend/resume wrong.  In
doubt, you can try without systemd, i.e. just by
   echo mem > /sys/power/state

This way, only kernel does its job without any extra userland hooks.

Hm, this reminds me of another good test: namely, try pm_test.  First, try like
   echo devices > /sys/power/pm_test
   echo mem > /sys/power/state

This simulates the S3, thus this must return quickly.  If this takes time,
something wrong in device suspend/resume.  Similarly, test like
   echo platform > /sys/power/pm_test
   echo mem > /sys/power/state
and
   echo processors > /sys/power/pm_test
   echo mem > /sys/power/state
and
   echo core > /sys/power/pm_test
   echo mem > /sys/power/state

If all these work (return quickly), the problem is really in the lower level.

You can find some details in Documentation/power/basic-pm-debugging.txt in
Linux kernel source tree.