Results of migration one chunk (default size: 64mb), collection with 180 million of rows

by {_id}:      ~ 245 secs
by {key}:      ~ 570 secs
by {key, _id}: ~1100 secs

where

_id - ObjectId
key - 32 chars - md5 hash

Using next configuration

3.75 GB memory
2 EC2 Compute Unit (1 virtual core with 2 EC2 Compute Unit)
410 GB instance storage
32-bit or 64-bit platform
I/O Performance: Moderate
API name: m1.medium

Even in the best case it is slow (245 secs)
No concurrent read/write to database, just sharding

"removeshard" is also not so fast

Attach logs (iostat, mongostat, mongotop, vmstat, at some tests have mongo

logs)

Related links:
https://groups.google.com/d/msg/mongodb-user/ziPjlJIS5_w/WMG3iZSuB6kJ
https://groups.google.com/d/msg/mongodb-user/R7XS1tDME8g/ucQR63ixmy0J