A53 slow down/lag/freeze after display replacement

Greetings ,

Yesterday i replaced a cracked screen for my customer's A53 that was not working since November of 2023 .

I found an OEM OLED Screen so i bought it replaced the old one .

After the repair was complete I turned the device on and ran the diagnostics where it passed all of them successfully, so I proceeded with gluing the back of the phone. I realized that the device lags quite a bit, so I suggested my client to proceed with factory settings because his mobile phone may have unwanted applications that make it slow. He took the device and proceeded to update the software and applications and noticed that the touch sometimes gets stuck, like when you try to browse your mobile phone with wet hands, something that is fixed by restarting the mobile phone, but it reappears after a while. It doesn't seem reasonable to me that the mobile phone is slow after a screen replacement, rather I believe that the mainboard flex is faulty and needs to be replaced. What is your opinion and what steps would you take in my position?