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":
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:
Affected users are encouraged to test the fix and report feedback.
This is in now, thanks! :-)