Hi Nandu,
Some flickering is normal. It is the result of the GPU changing to another display mode.
Changing display modes happens a few times during boot and also when using a lockscreen.
It is not a defect, it is a byproduct of switching videomodes.
Usually, on modern hardware you can hardly detect it, but on old hardware ( like your 2012 laptop) it is pretty visible. (It is also dependent on make and model of the GPU ofcourse)
You can, ofcourse, try other windowmanager/compositor combinations.
Maybe it helps a bit, maybe not.
It is very easy to switch: control-center -> mate tweak -> windows
