Other args given on the command line are expected to be databases, pgn files or search files. For databases, the file suffix ".si4" is optional.
Example: scid -fast mybase newgames.pgn will start in fast mode, opening "mybase.si4" and the "newgames.pgn".
Scid vs. PC's fast mode skips loading the spelling (or ratings) file, and recalculating name frequencies (as well as flag counts and estimated ratings). Scid vs. PC has recently reduced the bugs in name frequency handling, so using fast mode may now be fine, but is still not generally recommended.