Using stream_select() it will send http requests out in parallel. These requests can be made in a blocking or non-blocking way. Blocking will wait for the http response; Non-Blocking will close the connection not waiting for the response back. The API for httprl is similar to the Drupal 7 version of drupal_http_request(). As a bonus, a simple threading library is built on top of the parallel http requests functionality. This allows you to split a job and have multiple http "workers" running this split job in parallel. Anything that takes a long time to do can greatly benefit from using threads.
speed performance Drupal parallel