Apache Solr Parallel indexing allows you to set the amount of CPU's you have to utilise the maximum of your system during index time.
Improvement
During this test, entity_cache was enabled and pre-warmed. Solr server was an external server and the website was really complex. Looks like a realistic test scenario to me!
Without Parallel Indexing
time drush --uri=http://mytestsite.com.dev solr-index --limit=1000
1000 items successfully processed. 972 documents successfully sent to Solr. [status]
real 1m21.416s
user 0m45.394s
sys 0m2.748s
Node types: complex ones, references and more.
Amount per run: 100
Calculation : 1k nodes in 1m 31s, with 100 nodes per run
Nodes per second : 11.
With Parallel indexing
time drush --uri=http://mytestsite.com.dev solr-index --limit=1000
1000 items successfully processed. 1008 documents successfully sent to Solr. [status]
real 0m31.858s
user 0m2.948s
sys 0m0.492s
Node types: complex ones, references and more.
Amount per run: 500
Settings: 8 CPU's at once