Comment 4 for bug 988765

I had the same kind of problems (working with many video filess photo files, audio files, titles, image sequences, etc., all totalizing over 1Gb of source material and over half-hour of final video).

The problem disappeared when I put files on several separate, fast-access, USB keys (for example Patriot).
I tried the following configuration:
    - videos on one USB key
   - projects and thumbnails on a second USB key
    - other files (music, pictures/photos, sequence of pictures) on a third USB key

With that configuration, everything worked fast, almost instantly.

The downside is that if you do this in the middle of an ongoing project, you'll have the usual message "The following file(s) no longer exist", in which case you have to change each file property (the path part of the name) to reflect the files new location.