Since 2014 there has been a KDE bug report titled Ridiculously slow file copy (multiple small files). With the Dolphin file manager or other KIO-based KDE programs, copying many small files can be much slower than using the cp command directly or other copying programs like rsync, Finally that issue is in the process of being resolved for addressing very slow copy times when dealing with many small files on the KDE Plasma desktop.

KDE developer Méven Car recently took to addressing this long-standing defect in the KIO code of copying many small files being painfully slow. Long story short, Car worked on KIO to remove in-memory transport and batch-copy folding a whole run of files into a single command.

The code has yet to be merged but is expected for a post-6.29 release. From the performance numbers provided, the long file copy times relative to using cp are largely eliminated with the proposed code:

Méven Car remarked in his blog post:

"The small-files case is the story. KIO 6.28 was about 20x slower than cp, which is exactly the ratio the 2014 report complained about. Removing the socket for in-process workers, together with no longer probing the destination filesystem once per file, takes it from about 1.6 s to 0.4 s (roughly 4x). Batching the copy takes it to 88 ms, essentially cp speed and about 18x faster than 6.28. That "I use cp instead of dolphin" line finally has an answer."

Those interested in this significant improvement to KDE's KIO can learn more via this blog post.