I've been using Windows 7 for several years now, and always noticed that the copying speeds can be consistently relied on to drop throughout the task of copying files or folders. Today, I decided to test it.

I used five files ranging from 1GB to 8GB, copying them from one place to another on the same HDD - a SATA 3 (6.0 Gb/s) drive, but limited to SATA 2 (3.0 Gb/s, or 375MB/s) speeds because of the motherboard headers.

All of these five files started out copying at roughly 100MB/s, and proceeded to drop throughout until they'd eventually reached half of that by the end of the operation, with some ending up as low as 40MB/s.

Here's a benchmark for the drive:

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Although I was surprised at the low rates for the random tests, the sequential rates seem more than adequate to sustain 100MB/s for copying, which I believe is a sequential operation so long as the drive isn't fragmented.

Given this, why is Explorer's copy speed always decreasing from the moment the copy operation starts?

Is this simply an unavoidable quirk of Windows 7, or is there an issue somewhere that I can address to solve it?

Edit: The drive I used for the above tests and benchmarks is a 2TB Seagate Barracuda, but I've noticed this behaviour with Windows 7 across many other drives and USBs for several years, so I'm convinced it's an OS problem rather than a drive one. The reason I included the benchmark was to illustrate that it's capable of drawing those speeds.

  • In order to answer this, we need some more information about your harddisk. What kind of drive? How big? how much RPM does the drive has? What partition scheme is used? (fatex vs ntfs etc) – LPChip Apr 13 '18 at 21:49
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    Don't really need to know the disk type as I have wondered this myself - what is it doing to reduce the file copy speed? Only happens during drag/drop - Never during xcopy/robocopy – JohnnyVegas Apr 13 '18 at 22:40
  • xcopy/robocopy use different methods to copy that have their own strengths/weaknesses. – music2myear Apr 13 '18 at 23:14
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    why that speed is dropping ... In my observation this is because the first X bytes of the copy are artificially "fast" due to caching, but for files larger than the cache the average transfer rate continues to decline as more time passes. Keep in mind the fact the transfer rate is based on part on the the average rate so far, which by definition will decline over time if the transfer rate at the beginning was greater than the average rate of the transfer completed so far. This leads to a declining rate for the majority of modern storage configurations, since most of them include buffering. – Twisty Impersonator Apr 16 '18 at 2:04

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