When we create automation rules we can set to have them defer their processing to our background cronjob worker, which processes every two minutes, or we can disable defer processing to have the job process immediately.
Some jobs we will want to disable defer processing. For example, if our automation rule has an action to create a WordPress user account from a generated lead and automatically sign them into WordPress, we will want to disable defer processing so the cookies can be set into the browser immediately.
Also if for some reason we are targeting trigger actions that occur during a backend AJAX call, we can opt to disable defer processing in order to lighten the load of our backend cron worker. If we are running rules that occur every page visit and we have a high amount of traffic then we will not want to put that processing load onto our background cron worker. We will want to disable defer processing and process our rules immediately as our trigger fires.
On the other hand there might be a scenario where our trigger fires during a form submission. Deferring the processing of the rule to our cronworker can same precious resources, allowing the page transition to occur faster.
The decision to enable or disable defer processing will be a technical one. For most cases you will be okay leaving this toggle set to enabled.