How to make the transfer window closes when the data is really fully copied ? (windows fakes speed, and continue copy in background)

Windows normally uses up to 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 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.

This behavior is built-into Windows and cannot be changed.

Some file-copy products can get better speeds by avoiding using the Windows cache.

One such product is FastCopy, which is the fastest copy program I know.