I’m moving a bunch of files for some drive changes, and I’ve noticed a bunch of times that for 2 equally sized data transfers, the smaller and more number of files the slower it goes. Why does this happen?
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Storage on disk is a complex business: most files are broken up into smaller chunks (each piece having a link to the next one) and deleting files, which happens often enough, leaves gaps in the overall physical disposition of these chunks.
When you copy a file to that disk the operating system has to decide where to put the new chunks, and tries to be somewhat smart about that (e. g. finding an available gap to fill to not waste space).
So when you have a lot of small files it has to make that decision each time anew and there’s a noticeable overhead compared to a single big file processed in one go.
For similar reasons hard disks have to be defragmented (moving the chunks around on the disk platter) to get rid of the gaps, which is generally done continuously in the background while the machine runs. But not with SSDs where the data is accessed differently: they don’t need defragmentation. Which doesn’t remove the necessity to find free space for writing.
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