I discovered this when batch checking the MD5 checksums of a very large folder full of video files with md5deep : with the default options the analysis was multithreaded, so there were 8 threads with an i7 6700K CPU, and it was excruciatingly slow. It is both inefficient and potentially harmful : the mechanical stress is considerably higher when the heads are moving repeatedly across the platters to reach several areas in short succession, rather than staying at the same spot to parse a large contiguous file. Multithreading is typically not efficient when it comes to copying files from 1 storage device to 1 other, because the fastest throughput is reached for sequential reads, and using multiple threads will make a HDD rattle and grind like crazy to read or write several files at the same time, and since a HDD can only access one file at a time it must read or write one chunk from a file then move to a chunk from another file located in a different area, which slows down the process considerably (I don't know how a SSD would behave in such a case).
I wanted to comment a comment about multithreading, from 201510131124, but I don't have enough reputation points on Stackoverflow (I've mostly posted on Superuser up until now) :
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