10 second delay after pressing power button

Exactly the same here after upgrading to 10.12.4 on my late 2013 iMac like yours, but not quite as long a delay...ca. 8-9 seconds. Relieved to hear I´m not alone here! I´d already re-set the PRAM and even SMC as John Galt suggested...still a longer delay than before upgrading, but at least the thing then starts normally! I was beginning to think the power button was kaput. I reckon what Kappy has posted is what´s happening, at least I hope so! Obviously 10.12.4 just needs a bit more time to get going!

Hi! I have the same exact issue as your. Have you found a fix yet? Can you share with us?

I have two iMacs (late 2013) 27" with 32gb RAM, and 3TB Fusion drive. Both are running on latest Sierra 10.12.5. They both perform great, fast, and reliable since I bought them in December 2013. I've been very good at updating them whenever there is available update. Only about 2 weeks ago, I noticed that one of them, when I press the power button on the back, the screen would light up (black) and would get stuck there for about 15 seconds before the fan would start up and the startup chime would play, and the Apple logo would come up, then it would take about another 15 seconds (which is normal) to reach the desktop screen. Then everything works just as it should. Restart is normal. This issue is ONLY when you turn on the iMac using the power button.

At first, I thought that was strange that there's a delay in hearing the chime when it used to be almost instantaneously after pressing the power button. What's baffles me is that the other iMac with same specs and OSX installed is not having this issue. Thus, this is not software related, rather a hardware somewhere in the BIOS/POST.

What I did so far which made no difference:

1. Select Startup disk and restart

2. SMC reset

3. PRAM reset

4. Swapped RAMs with the other iMac

5. Safe mode

6. Diagnostic Tools reported no issues found

7. Check potency of the internal battery (turned off wifi, unplugged iMac for 3 days, plugged it back, turned on) it kept date/time which means the battery is in good shape.

This is very frustrating! Any suggestions would be very much appreciated. Please HELP. Thanks!

Downgrading mine did not make any difference. I was able to isolate the issue - it's not present if I'm on SSD alone. But when I plug the HHD back (original apple fusion drive with the ssd) that's when it delays again. It gotten worse latetly. I now have 18 seconds delay when it was only 8. I think the issue lies in the fusion drive. Do you have fusion drive?

An SMC Reset may be justified: Reset the System Management Controller (SMC) on your Mac - Apple Support. Try it.

The system is verifying memory and other items essential for the startup. Happens on mine, too.

Same here. Have you found a fix?

I've just developed this same issue.

Downgrading the the version of the OS that came on my iMac fixes it. Upgrading back to the latest version causes the problem to return.