Question [Q] LG Optimus G Pro LG-F240l

Hello, I just exchanged my Galaxy S3 i9305 with had 4.2.2 installed with miui rom. It's pretty damn laggy and I checked with CPU-Z everything thing is ok but in the core part where it says core 0 core 1 core 2 core 3 only the first core is showing 1728 MHz the rest of the core says "Stopped"...on my s3 all the core were running...why? Also when I try to take picture with HDR it says "Camera Stopped Unexpectedly please restart" is that rom fault?

Also I want to downgrade my phone to 4.1.2 because I need to run "Screencast Pro" which only works on 4.1.2 so could any please tell me exactly which one to download? and how to flash it?

*This is my first time with LG mobile so I'm kinda new to everything Thanks.