Can I run a slow action in a seperate thread?

Techcrunch has released a library to spawn an asynchronous task within WordPress.

https://github.com/techcrunch/wp-async-task

Essentially, you can take any process that is triggered by an action hook, and you can defer the processing on that hook to run asynchronously. You extend the class to define which action you are triggering and a couple of functions to prepare_data to pass, and the action to perform.