The first thing to say is I don't know where to write this. It has to do with Dolphin but not exclusively.
I'm taking the last steps to switch to Linux.
I wanted to convert 3 big HDDs from NTFS to EXT4, for which I have used 4 HDDs that I have for backups.
All transfers use internal HDDs with SATA3.
My system is Manjaro KDE 18.0.4 just installed and without modifying or updating anything.
I formatted the 4 backup HDDs using EXT4.

To fill the first HDD I used Dolphin and cp command from terminal.
It was a nightmare. It took me 3 days to copy 7 TB.
The speed went from 150MB/s to 0 going up and down.
Sometimes the process got stuck and I couldn't stop it or kill it. I had to reboot the system completely.

With the second HDD backup, the same thing happened. Horrible!

For the third backup I used Ubuntu Gnome in a Live Session with the program Files.
Everything was perfect, very fast and without problems.

Then I realized that the space of the HDDs formatted with EXT4 was much smaller than with NTFS. I investigated the matter and discovered that the tune2fs utility solved the problem. I used it in the 4 backup HDDs.

Then I went back to Manjaro KDE and this time everything was very fast.
I filled the space that had been left free in the 3 previous HDDs and finally transferred what was missing in the fourth HDD.

The last step was to format the initial 3 HDDs using EXT4 and re-copy everything from the backup drives.
This transfer was from EXT4 to EXT4 and logically it was all perfect.

Is this a bug? Something I did wrong?