Lightroom CC 2015.7 now available
http://blogs.adobe.com/lightroomjournal/2016/09/lightroom-cc-2015-7-now-available.html
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Smart Previews for Faster Performance
You’ve always been able to use Smart Previews in the Develop Module as a way to edit photos without having access to the original images on disk. Astute customers reported faster performance in the Develop Module when using Smart Previews and keeping their original images disconnected from their computer. Now, you can set a Preference in the Performance tab so that Lightroom will always use Smart Previews in the Develop Module, if they are available, even when the original files are also available.
To do so,
Choose Edit > Preferences.
In the Preferences dialog, select the Performance tab.
In the Develop section, select Use Smart Previews Instead Of Originals For Image Editing.
Click OK and then restart Lightroom.