The Ultimate Guide to FCP X Proxies
Proxies give you tons of flexibility – they increase playback and editing speed without forcing you to sacrifice quality, allow you to edit seamlessly on laptops and portable drives, and provide a streamlined project delivery, whether that includes a handoff to a mixer or colourist, or exporting a full quality output for a client. And FCP X makes all of that simple.
Source: https://blog.frame.io/2017/05/11/ultimate-guide-to-fcp-x-proxies/