I assume you probably are, but the safety of assumptions has already been mentioned...
Are you guys running any sort of light/exhaustive diagnostics on these drives that go slow?
I've seen drives which slow down for no apparent reason, and once formatted quickly go slow again... However there is consistently a trend of bad sectors and errors on the drive showing signs of failure.
Basically this has been my experience with HDD's:
A) The drive works fine.
B) The drive has problems, slows at times, and fails diagnostics (this is a crapshoot, diagnostics often pass when the drive has obvious signs of failure)
C) The drive has problems, slows at times, passes diagnostics, but scandisk fairly consistently finds errors or consistent (relevant to time) errors in windows (sign of failure, indeterminant amount of time)
D) The drive has problems, and bad sectors, failure very emminent.
Now Larva's experience could be something that could explain a drive just going slow, but with circuitry on the controlling board going bad... How can this propagate in the form of the drive just operating slower than normal, but avoid any other signs of failure in the quality of written data? That would seem very tough to technically explain or rationalize.
Now, I have seen drives which emit a clicking sound when they are going bad, with very few symptoms other than the system almost becomeing completely unresponsive for the duration of the clicking - I presumed that this might be due to a loss of control over the arm. This is the closest thing I have seen to a drive just going slow, however it could not by any means be considered normal operation, and always means failure in a matter of months.
As a mechanical part, it only makes sense that the HDD would show signs of wear and tear like anything else, and possibly slow down over time... But it also makes sense that by the time this becomes readily noticeable it would also almost certainly cause errors on the drive as it would become harder for the drive to write and read consistently where it should when the mechanics are not working correctly.
I guess it appears as though "going slow" might be possible, however an extremely rare symptom of a hard drive having trouble.
@larva: Do you have a low opinion of Storagereview? From my experience reading there, they do one of the best jobs at reviewing drives anywhere, and they have some basic documentation on many things that is some of the best, though somewhat dated at times.
@PhobMX: We all know drives slow down, however, I've never seen a hard drive with a decent install (not older than crap and software bogged) just perform noticeably below its normal level, without accompanying signs of abnormal operation.