I encountered a strange case with a notebook that was really slow now and then, especially noticeable when writing something on its keyboard (like a simple web address in the browser).
It's true that it was badly treated and it has a broken corner due to a slip out from its case, associated with free falling from 1 meter or so. However, other than this slightly broken corner everything run apparently fine. Tried de-virus, de-frag, a successful hard disk thorough check (WD Diagnostics), almost reinstalled window$ but no change.
The explanation sprung naturally when the user noticed that, when playing movies (from hard disk) if shaken, the playback will be interrupted and then automatically restarted when stopped shaking it.
In other words there is something probably broken inside the hard disk that makes the whole system wait and request a re-reading of the current data if notebook shaken. I'd say is a damper broken or so.
Why lingering when typing?? Because of the small shocks from the fingers, the original user being an impetuous email writer.
Oddly enough, if shaken during the thorough WD diagnostics testing, it stops and waits for device to become ready each time it's shaken, however in the end the test result says everything is ok.