Feature request

Description of problem:
Docker does not extract images in parallel

docker version:
Client:
Version: 1.10.3
API version: 1.22
Go version: go1.5.3
Git commit: 20f81dd
Built: Thu Mar 10 15:54:52 2016
OS/Arch: linux/amd64

Server:
Version: 1.10.3
API version: 1.22
Go version: go1.5.3
Git commit: 20f81dd
Built: Thu Mar 10 15:54:52 2016
OS/Arch: linux/amd64

docker info:
Containers: 23
Running: 0
Paused: 0
Stopped: 23
Images: 15
Server Version: 1.10.3
Storage Driver: aufs
Root Dir: /var/lib/docker/aufs
Backing Filesystem: extfs
Dirs: 303
Dirperm1 Supported: true
Execution Driver: native-0.2
Logging Driver: json-file
Plugins:
Volume: local
Network: bridge null host
Kernel Version: 4.2.0-30-generic
Operating System: Ubuntu 14.04.4 LTS
OSType: linux
Architecture: x86_64
CPUs: 4
Total Memory: 1.945 GiB
Name: something-else
ID: 37YL:PW3M:M5GE:PSQ5:TMHR:QCFI:M64D:53RJ:2LOA:CLU2:BXUR:X2RG
WARNING: No swap limit support

uname -a:
Linux something-else 4.2.0-30-generic #36~14.04.1-Ubuntu SMP Fri Feb 26 18:49:23 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

Environment details (AWS, VirtualBox, physical, etc.):
physical

How reproducible:
very easy

Steps to Reproduce:

1. docker run some image you dont have
2. notice the images are extracted one by one. This could be Parallelized

Actual Results:
images are extracted one by one

Expected Results:
images are extracted in parallel

Additional info:
None