[Borgbackup] recreate --recompress incredibly slow

[devzero at web.de]

Hi, 

i'm trying to get a guess how much space i would save with transition from lz4 to zstd, so i'm converting an 15GB archive like this:

/backup/bin/borg-1.1.4 recreate /iscsi/lun1/borg-repos/host1 --recompress --compression zstd

the process is incredibly slow, so slow that i possibly cannot convert all existing repos (despite the danger...). maybe i need to start from scratch...

can someone explain why it is so slow (maybe it's by design?) or is there room for optimization ?

regards
roland