MongoDB - Insertion Speed
1) Add an index will slow inserts.

2) It will speed up inserts

3) Inserts don't use indexes

4) It will speed up writes but w=0 risk consistency and j=0 risk durability

5) You can't insert data to secondaries, insert/update/delete applies only to primary

If you care for insert speed you should use hash based sharding which evenly distribute writes among shards and increase write throughput. (http://docs.mongodb.org/manual/tutorial/shard-collection-with-a-hashed-shard-key/)