Performance of External Hard drives

I am getting a Zotac Zbox EN760 for light to medium gaming. It only supports 1   2.5'' drive and while I would like to have an SSD for boot times and other SSD related advantages (OS, key programs, and games ), this doesn't leave me a lot of storage for movies, files and other programs. How much of a deficit as far as performance will I take by not having a standard HDD, but instead getting a comparable USB 3.0 based drive?

 

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http://www.tomshardware.com/forum/297224-32-speed-external-usb3-internal-sata