iPhone 8 + Black Screen During Boot - Running iOS 11.1.2

Hi. Last week I purchased a new iPhone 8 + to replace my iPhone 6. The tech at my local Apple Store told me I needed to update my iPhone 6 to the latest iOS before backing up my iPhone 6 to iTunes and restoring it to a new iPhone 8 +. Foolishly, I listened to him and updated the software on my iPhone 6 to iOS 11.1.2 and backed the iPhone 6 up to iTunes on my MacBook Pro. A few days later, I went to a different Apple store hoping for better service and found very helpful people at the Apple Store in Shadyside in Pittsburgh, Pennsylvania. They told me I did not need to update the software on the iPhone 6, but the deed was already done. When attempting to restore my new iPhone 8 + to the backup of the iPhone 6 on iTunes, the restore would not initialize since the iPhone 8 + was running iOS 11 out of the box. The tech then updated the iOS to 11.1.2 manually on the iPhone 8 + through a way I was unfamiliar with and then successfully restored my new iPhone 8 + to the backup of the iPhone 6 on iTunes. Unfortunately, a few days later when I power cycled the new iPhone 8 +, I discovered that when the phone was powered on I would receive the normal white screen and black Apple logo for about 5 seconds, then the screen would turn completely black for about 5 seconds, then the normal white screen and black Apple logo would return for about 5 seconds before finally displaying my lock screen. Furthermore, I noticed that if I unlocked my iPhone 8 + as soon as it was turned on, the entire phone would be laggy for about 10 seconds. However, if I let the phone sit at lock screen for about a minute or so after booting on, I would experience no lag when entering my passcode and using my phone. Finally, I notice that my new iPhone 8 + experiences keyboard lag when it has less that 5% battery power remaining. I have contacted Apple support about this problem and spoke with 2 different Apple senior advisors. Remote diagnostics were ran twice on the iPhone 8 + and all came back normal. They did not know why the problem was occurring but stated that they knew it was not hardware related but said that they would escalate it to Engineering. They stated that I could try to reset the phone and then restore it to an iCloud backup, however they did not know if that would resolve this booting conundrum and I myself stated that I did not want to do a reset right now since I am currently out of town at a location with limited internet and probably would not be able to successfully restore the iCloud backup from my location. Anyway, I am reaching out hoping that someone out there would know what is causing the short black screen during boot and the lag directly after booting. Could it be an undetected hardware issue? Is iOS 11.1.2 to blame and will the newest update fix this problem? Are these issues due to corruption in the iTunes backup from the iPhone 6 that was used to restore the iPhone 8 +? I personally am leaning to the possibility that iOS 11.1.2 is to blame because my sister just bought an iPhone 8 + as well but restored it from her iPhone 6 running iOS 10; so she is only running iOS 11 on her iPhone 8 + and is experiencing none of the problems that I am having. However, other than the short black screen at boot, lagginess directly after booting, and keyboard lag when at less than 5%, battery, my iPhone 8 + is operating perfectly. I am just wondering what is causing these problems, will they get worse, and how will they be fixed. Thanks in advance.

Tom

P.S. Even though my iPhone 6 is running iOS 11.1.2, I am unsure if it is experiencing the same problems as my iPhone 8 + because I don’t remember watching it boot and because I am out of town and do not have the iPhone 6 with me.