Yeah it might be, but as I said, it's only the first "move" that lags, it record the touch at the exact moment I touch the screen and after the first 1/2 inch is past there's no problem anymore so I don't really think the problem is related to that.
Actually, I think there was a similar "problem" a few years back, it was related to the way Android make a difference between a click and a swype. Its create a deadzone around the first point that's touched by your finger, so that every time you click it's not considered as a swype (because biologically you just can't click only ONE pixel). The size of this deadzone was the same on every phone and was related to the touch_slop value. At some point on some "newer" phone the size of the digitizer was bigger but not the dpi and so the deadzone began to be a hindrance more than a conveniance, cause its size grew with the size of the screen. I was then decided to change the way it was handled by the system and a new method has been implemented, using mtouchslope value, that's not just a number of pixel but that's calculated from the dpi value of the digitizer. SO I think that the digitizer I got isn't the same as the stock one (just like you said) but I think there's a way for me to fix the problem in a more "software-way". The only thing I don't know here, is how can I change the digitizer's dpi value that's stored inside the phone.
Before : Xperia Play R800i - Xperia NXT 2 - Android 2.3.7 - Rooted - Unlocked BootLoader - DoomKernel v14
Now : Xperia SP C5303 - eXistenZ Ultra 4.0.0 - Android 4.3 - Rooted - Locked BootLoader