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GillesH:
FFMS2 and L-SMASH Works do the same for AviSynth as the FFmpeg input plugin does for VirtualDub: Using the core of FFmpeg to read media files and provide their content to the plugin's host system.
Building a keyframe index is nothing bad: It happens only once, and it adds another layer of reliability for the case that frames from the source clip are requested in a random order, instead of sequentially; sometimes the internal keyframe index in a container may even be damaged, and a source plugin relying on them would fail. The plugins do that additional indexing for your convenience. And L-SMASH Works offers two different technologies: LwLibav*Source uses an additional index (but supports a lot of containers, even those which do not have an internal index at all), and LSMASH*Source relies on an internal index inside the clip (but supports only a few containers).
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