If you read all posts then you would notice that some who had those HDs replaced "started" seeing the issue after a few days.
My take on this, it is always there. It's a placebo effect for those who just got the replacement. Some are kind enough to mention it here, some are... well maybe just upset that they went through all these troubles for nothing.
Also in many posts here you can see that people describe the same effect seen on their laptops. In my opinion this not a defect it's just the way it works.