Very erratic file transfer speeds in Windows 8.1

Windows normally uses about half of the memory for caching disk files.

This means that when copying, Windows at first reads and writes very fast the data, but that is an illusion since the data is only copied to memory. When the memory cache is full, Windows then has to wait for some writes to the disk to physically terminate in order to liberate cache space, which is when the slow-down starts to be felt.

The copy operation at this point will be done in spurts: Reading quickly to memory, then wait, repeating itself again until the copy is finished. The displayed speed in this phase depends on the amount of memory of the computer: The more memory there is, the more visible are the spikes.

In case of network copy, as the physical write is slower and as it might take longer for full speed to be attained, there are many more variables that can affect the copy speed, pertaining to the nature of the connection.

This behavior is built-into Windows and cannot be changed. Some file-copy products claim to be able to copy faster than the Windows copy, but the ones I tried only seemed marginally faster. If you wish to try some of them, see : Best Free File Copy Utility.

To finish, here is one famous cartoon: