Mounted volumes extremely slow


On the current version of Rancher OS (v0.7.1) I use a container that manipulate a lot of large files (4Gb or more) in a mounted volume.
All operations (copy, move, etc…) are exxxxxtremely slow (ex : 5 minutes to move a 4Gb file).

Here is an extract of my docker info result :

Server Version: 1.12.3
Storage Driver: overlay
 Backing Filesystem: extfs
Logging Driver: json-file
Cgroup Driver: cgroupfs
 Volume: local

Any idea ?