When copying heavy files from an ext4 to an ntfs partition using dolphin, it eats up all the system resources in the process, slowing down everything else. The same thing doesn't happen when doing the same with caja.
What system resources is it using CPU? RAM? Also what do you mean by heavy files, is this large file sizes? A large quantity of files?
Both CPU and RAM. Happens when any of the files to copy is large.
NTFS drives are notoriously bad for performance, the driver for it in my opinion (not KDE) has always been poor. I just copied a 3.3GB file to an external HDD that is NTFS and it used most of 1 CPU core. What are the hardware specifications of your PC? CPU Model, Amount of RAM.
Back when I reported this bug, it was an Intel Pentium M (4 cores, around 1.6Ghz) with 8Gb of RAM. Tried again today with a high end CPU (Ryzen 7 2700, 8 cores, 16Gb ram) and a more modern KDE frameworks version, and it seems resource consumption has been improving (it doesn't manage to overload this CPU), but it's still quite high compared to gnome software.
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KDE software doesn't handle NTFS separately, if this is slower than other file systems, then the issue needs to be investigated by NTFS developers.