I had this slow boot up and slow app opening problem where the iphone 7 plus also had no sound after a screen change. I paid an extra $30 for a "premium" screen because this was my wife's iphone. It came with the complete ear speaker and proximity /front camera ribbon and heat sheild already installed. All I had to do was swap the home button to the new screen. I did the same as any veteran tech would do with batt disconnection before removing any screws or cables. But it created the same problem and the back of the phone became warmer than normal.
I wanted to add that I swapped back all the original OEM parts with the broken screen back on the phone and it acted normal in boot up and response to touch and sound. It operates at normal temp. It contains ISO 11.0 on the software. This was present before and after the screen swap.
I know many did not have this success when trying to trouble shoot this issue BUT either I got lucky or hopefully this will create a lead to solving this unknown issue.
Thanks for the posts everyone!