Why does moving large folders take a lot of time?

It happens when I use Windows Explorer to move (or to cut and paste).

The only way around this that I know of is to use something other than Explorer to move the directory. For example from Windows' cmd.exe, using move a b moves even large directories instantaneously. The same goes for Cygwin's mv command.

EDIT: Some speculation: Explorer may immediately update the search index, which takes time if there are many entries to update.

When a folder appears to move instantly, it's because the operating system has been able to update the file allocation table* without moving the actual file data.

It's slower to do this with small files, as for each one a change has to be made in the table. If the files are really small, this may even take a similar amount of time to actually moving their data.

I don't know in exactly what circumstances the file data has to be moved when it's on the same partition as its destination but I would assume there's nothing you could do to avoid it. As other commentators and answers have hinted at, copying to a different drive (different disk, different partition, to a USB stick, to a drive over the network, etc) would of course mean you had to copy the full data, so things will be slower. You will be affected a lot more by the bandwidth of what you are copying to.

(*Think of file data as books in a library, and the file allocation table as a set of index cards showing you which section the books are in)