Yes

Then multi-(thread/processing) your problem will not help (much). Adding contention between reading and writing will probably slow things down

And alternative strategy: separate the reading and writing.

First pass reads the files, extracts the relevant field and construct a hash mapping original path/filename to new path/filename.

Second pass reads the filenames again using opendir. That (should) give you the filenames in whatever order the filesystem considers its native ordering. That might be alphabetically sorted, or it might be order by creation date. Whatever, it should be the fastest way to access the on-disk directory structure.

Process the filenames in whatever order the OS/opendir gives you them in. look up the original name in the hash to find the new name, and use rename to move them.

Rational: separating the reading and writing removes contention at the hardware level; renaming in the same order the OS gives you them, reduces inode/FAT32/HPFS cache misses.

Moving (renameing) a file does not cause any (file) data to be duplicated; it is simply a change to a field within the filesystem directory structure. Making that change in the same order the filesystem gives you the names, ensures that the modification is made immediately after the inode/... is read, therefore still in cache, saving a re-read/cache miss et al.; and should be the fastest approach. The filesystem LRU cache is optimised for this case.

HTH.