1.75 Millions files, is "really too many files", personally I´m sure your copy speed can be much more faster if you reduce drastically number of files.
Have in mind, before OS accepts the file it should check if name is not duplicated, choose which sectors are available on disk, write file name in directory tree and consider disk fragmentation, so there is a lot of work behind scenes. So it needs to find an available slot before get a file stored.
In addition, I would consider IIS have more priority than a user trying to copy files