Asynchronous processing
Salesforce is committed to ensuring that the interactive user experience (browser or mobile response times) of your applications and that of its own is as optimal as possible. In a multitenant environment, it uses the governors as one way to manage this. The other approach is to provide a means for you to move the processing of code from the foreground (interactive) into the background (async mode). This section of the chapter discusses the design considerations, implementation options, and benefits available in this context.
As the code running in the background is, by definition, not holding up the response times of the pages the users are using, Salesforce can and does throttle when and how often async processes execute, depending on the load on the servers at the time. For this reason, it currently does not provide an SLA on exactly when an async piece of code will run or guarantee exactly when the Apex Scheduler code will run at the allotted time....