How your computer is being taxed by running other programs. Your computer can only handle so many tasks at once, so when you have multiple programs running it sometimes has to juggle all those tasks, causing slow downs in your copying.
It also has to do with file size, if you’re copying a bunch of photos for example, ones that have smaller files sizes will copy quicker than those that are larger, but if you’re just looking at a “photo X out of 100” progress bar it just seems like some are randomly causing slowdowns.
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