Import mailbox too slow

[agm33]

Hello everyone.

I'm importing mailboxes using zmmailbox:

# zmmailbox -z -m ${account} pru "/?fmt=tgz&resolve=replace" user_mailbox.tgz

It works fine but performance is terribly slow, like, 10MB/s! Wich will take several days for a 4TB user base.
The server has a lot of CPU and memory, and no issues reported.
Any ideas on how to improve it?
Thanks in advance!!!