P: CC jumps to front when opening files in the background.

Right. No resolution. I wasn't noticing the issue during startup so much as when I was using image processor to process thru whole folders or groupings of images. PS would steal the screen and leave the rest of my workflow paralyzed until it completed the processing. The only resolution was to revert back to the older version of PS so now I'm stuck paying monthly for non-current versions of the program. Now that I know this is happening both on PC and Mac and that it started on introduction of updated versions and resolves when rolled back to 2014-15 versions, how Adobe can maintain that it's an internal permissions issue - I have no idea. But no, I'm not anticipating a response from them here.

It happens on PC too. With every newer version Photoshop steals more and more time at starting. Now it bacame absolutely insane. 

Cool. Thanks Chris.  So Adobe can't point the finger at Apple. 

This is Windows too.

BTW, does this happen on Windows as well or is it a Mac specific thing? Certainly drives me insane...

This needs to be fixed. How is this not an issue for Adobe? It is a nightmare to start up Photoshop.
It has been an issue for 3 years and no word from anyone at Adobe?

With Photoshop CC 2017 things got even worse. Adobe couldn't care less about users comfort. Too sad. 

Because Photoshop's startup can take some time, I like to work on other
tasks meanwhile. Unfortunately Photoshop is shifting during the starting
process several times to the foreground, which every time interrupts
the work on other things and can be quite annoying. Is that really
necessary? It would be great, if the program simply loads in the
background.

Have this suddenly happening to me, running on Mac, with latest update. Two things - I've rolled back to previous version and that stops it from happening. But that's not a great fix. Thing two - before I rolled back, Adobe asked me to run photoshop under a different user account on my Mac. When I logged out of my user account and logged in as a guest user, the update version didn't do that and worked fine. Therefore Adobe conclude that the issue was an Apple issue, not an Adobe issue, and that I should contact Apple to troubleshoot - that the issue may be with Mac permissions. Doesn't explain why the earlier version still works without hijacking my system. Don't know if this is helpful to you or not -

Adobe Lightroom does the same annoying thing, about several times during its start up, on various versions of Windows.