When you use the Build Preview Movies command low resolution preview movies are created from your source movies to use even when the original source media is offline.
The following compression presets are available. The approximate file size for one hour of preview footage at 160x120 resolution is shown in each case. (In the Professional Edition you also have a full frame rate Offline RT preset, and can create your own customised preview settings using any available QuickTime compressor.)
The difference in time to compress between the speed optimised and normal presets can be a factor of five or more, but with a corresponding difference in the resulting file size and quality.
Note that you should not access the preview files from the preview directory directly. They are private to CatDV - this is why they have obscure names like "25,123,4210,64x.mov". Instead, use the Export As Movie(s) command, where you can choose to export existing previews as either reference or self contained movies without recompressing them.