Currently all the thumbnails are fetched on the startup which takes a considerable amount of time; they should be fetched when idle at some later stage.
[talking about the thumbnails of "parts" in the viewer - eg. the slides]
Tomaž Vajngerl committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=c6fd07e0e46fa6ecba38cf7bd2f0e0312eaf7600 fdo#85849 on-demand create thumbnails of tiles in the sidebar It will be available in 4.5.0. The patch should be included in the daily builds available at http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: http://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.
This is in now, thanks! :-)