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When you select some files and copy them, the file copying dialog appears with progress bar and speed indicators. But when you select some other files and copy them, another dialog pops up and both seemingly show more than 50% speed slowdown as compared to only a single file copying dialog. I'm fairly sure that back in the days doing this with a floppy disk actually did slow down the process significantly. But is it the case with HDDs and SSDs in modern Windows OSs?
Does copying several batches of files through multiple file copying dialogues slow down overall I/O operation in Windows 7?
For example, let's copy a hundred of 50 MB files from HDD A to HDD B and right after that copy another hundred of other 50 MB files from A to B. So the files aren't small and the slowdown isn't caused by file table stuff issues due to small size. Both disks are 90% empty and contain no fragmented files, so there are no problems coming from that way too. Disks are connected via SATA, and aren't external.
there is a lot to this question that is hardware dependent. for instance if you are copying to a usb2 device with a IO speed of 35MBps, you will have to divide that IO rate by the number of processes. if your copy involves multiple small files, it is more likely to cause conflict when maintaining partition metadata, than a pair of large files, etc. – Frank Thomas – 2013-11-26T13:29:15.130
1Another thing to consider is, are you copying a bunch of smaller files or a bunch of larger files. Copying smaller files is inherently slower on most operating systems than copying larger files. For example, if I copy 5GB worth of 20MB/each images, it will usually be much slower than copying a single 5GB movie. In my experience, parallel copying of large files is nearly as fast as doing two separate copy operations serially, whereas a bunch of smaller files being copied in parallel is MUCH slower. – Trav – 2013-11-26T14:01:26.730