After several years, my Samsung GS5 suddenly started running the battery dead in a few hours. Just days before, the battery was lasting all day before it needed a re-charge. Coincidentally, I had just purchased a new phone that was on the way from T-Mobile with a new, unactivated sim-card. I have no way to prove that anything was pushed to the GS5 to make it start acting badly, but the timing was extremely suspect.
So, I still had plenty to do on the old phone to transfer on-phone data to the new device, so I still needed it to be working. I was wondering what was causing the excessive battery drain, so I opened up the "#top" app to see if there was a process sucking up lots of CPU time. This is where I noticed that a process named "system_server" was constantly running at 30%+ CPU. Searching for answers about what "system_server" does was a fruitless pursuit. There was some noise about turning off bluetooth, or making a call to voicemail to reset something, but all of the internet voodoo related to the system_server high cpu issues changed nothing.
A few days later, I started the process of clearing off apps from the old phone one-by-one, in hopes that one of them would reveal itself as the guilty party. Nothing seemed to relieve whatever was going on within system_server though. Then I uninstalled some app I had installed ages ago, and I couldn't even say which one it was because the next thing that happened was SOOOOOO infuriating and distracting. The phone suddenly started displaying the following message, over and over and over. "Unfortunately the process android.process.acore has stopped." I would tap the OK button and the message would pop right back up. I had about enough time between each pop-up to tap one other thing. Restarting the phone didn't make that go away either.
More internet searching gave me a suggestion to open the settings, switch to the "Applications Manager", find the "Contacts Storage" app (which only appeared in the "all" applications, and the "all" category wasn't even on the screen at first, but I digress...). I then cleared the cache for the "Contacts Storage" app, but that didn't make the messages stop. I then tapped the "Turn off" and "Force stop" buttons and the message finally stopped popping up. Just to be sure it wasn't a coincidence, I tapped "Turn on" and the messages started popping up again immediately. So I turned it off and force-stopped it again to end the insanity until I could find the real cause.
The interesting thing about all this is that after I stopped the "Contacts Storage" app, the original issue with the system_server process burning up the CPU was gone. system_server was now running at only about 1% CPU and the battery was back to its normal discharge rate.
There was so little information to be found that would have linked these issues together, I decided to write this in case it helps connect the dots for someone else who is dealing with this torment. Hope it helps.
This helped me: https://www.quora.com/How-do-I-fix-the-process-android-process-acore-has-stopped-error