Speed up Automator workflow
Try this alternative approach in Automator. Be sure to back up any PDFs before applying this workflow – the originals will be overwritten.

In Automator.app, create a new workflow and add the following actions:


Ask for Finder Items, with Allow Multiple Selection
Apply Quartz Filter to PDF Documents, selecting the Reduce File Size option



When run this workflow will let you select multiple PDFs and then overwrite those files with reduced file size versions. It performs the equivalent steps as the original workflow, but is faster than scripting Preview.app.