Mongodb: slow shard balancing
Math first: you moved 150M documents in 2 days, which is roughly 860 documents per second including metadata and indices, where reading and writing all occurs on the same machine. That is not what I would call slow. The description coming to my mind is "lightning fast". ;)

Since there is no real distribution of the write load, an easy way to speed things up is to add two or more machines.

A few notes: sharding production data on non-replica shards is dangerous, to say the least. If one of the shards fails, the data contained is permanently unavailable until you get the shard up and running again. Plus, since there was no server to write to for a specific key range, values of that range can not be written. If the shard was a replica set, the failure would lead to the election of a new primary, to which all writes and (depending on the configuration) most or even all reads would go.

_id can be used as a shard key, however it should be hashed.