Default Lags on d802 stock rom on apps

Hi all,

I have d802 with stock rom, the phone is not packed with apps and has maybe 5-6pics on it.
The phone is running 4.4.2 kitkat and a lot of times when I open the browser and get to a site (nothing specific it can be any site) the phone freezes and the screen stops to respond,this happens a lot when I'm opening a PDF file.

The only way I can get the phone to start responding is to lock the screen using the button and wake the phone up and open the screen.

Any one knows why this is happening? It's frustrating! My old iphone 4 didn't get stuck as much as this 2g 700cores phone.

Appreciate your help.