You can set Robocopy to use multiple threads, (Win7+ ; by default it is 8), having said that, most likely the problem will be that all the operations are being performed on the same source and destination concurrently which probably leads to a bit of head thrashing on a HDD.
You could try reducing the number of threads Robocopy uses with the /MT:x
parameter, (x is the number of threads required between 1 and 128).
Possibly start at /MT:2
and increase by one until you see performance drop off.
mkdir isn't needed for Robocopy, it'll create folders as it goes and it should skip any files that have already been copied (options permitting) according to the table here
BTW, if you're going to use Robocopy where there's any chance that the files will already be opened then it will fail on those files, (it won't access open files). You can get around that by either using:
(which is a replacement for Hobocopy below) which will create a VSS copy before running Robocopy, or;
which was a less featured version of Robocopy that could use the VSS, (written by the author of ShadowSpawn).