This mode delegates the work done by the back end to a different thread. That way, throughput and response time are (to a certain extend) decorrelated from the back end performance. The drawback is that a small delay appears between the transaction commit and the index update and a small overhead is introduced to deal with thread management.

It is recommended to use synchronous execution first and evaluate asynchronous execution if performance problems occur and after having set up a proper benchmark (ie not a lonely cowboy hitting the system in a completely unrealistic way).