Render preview images
Aperture renders preview images in four situations:
When a library is upgraded—provided that the “Create previews for existing images” checkbox is selected in the Welcome to Aperture dialog
When a photo is imported, or a version is created, rotated, or adjusted—provided that the Maintain Previews item is selected in the Library Action pop-up menu
When you select the library, a project, or a photo selection and choose Photos > Update Previews
Immediately before a slideshow, if the previews are not up to date, and the slideshow preset uses the Best quality setting
When rendering a group of preview images, Aperture uses the current application preference settings for compression quality and picture size, not the settings that were specified when preview rendering began. For example, if you have the quality set to 6, and then halfway through rendering you change the setting to 8, the previews that have not yet been rendered will be compressed at setting 8. Previously rendered previews are unaffected.
Update previews to use new size and compression settings
In the Browser, select the photos you want to update.
Hold down the Option key, then choose Photos > Generate Previews.
If you encounter performance issues, you can check the Activity window to see if Aperture is doing work in the background (choose Window > Show Activity). If you think that preview processing may be degrading performance, you can turn off automatic preview maintenance for that project and cancel the preview processing task underway. You can then generate the previews at a more convenient time.