Here is my last copy benchmark:
I have two Nvme SSDs, each SSD has Btrfs and Ext4.
Btrfs: Copying the game “Dota 2” directory without archive from Btrfs SSD A to Btrfs SSD B
$ time cp -r "~/Desktop/dota 2 beta" /run/media/zesko/Backup/dota2_backup
real 0m33,970s
user 0m0,117s
sys 0m26,751s
Ext4 Copying the same game directory from Ext4 SSD A to Ext4 SSD B
$ time cp -r "/media/Steam/steamapps/common/dota 2 beta" ~/VMs/dota2_backup
real 0m39,838s
user 0m0,105s
sys 0m30,614s
Therefore you cannot trust what kdiskmark showed but it is not for real world use.
Just forget about kdiskmark