When you start Scid from a shell or console, there are command-line options you can specify. Scid-format databases (with or without a file suffix such as ".si4") and PGN files to be opened can be given (for example)
There are also optional arguments to control which files Scid should search for and use when it starts. You can turn off the use of tablebases with the -xtb (or -xt) option, avoid loading the ECO openings classification file with -xeco or -xe, and avoid loading the spelling file with -xspell or -xs. Also, the option -fast or -f does all three, so scid -f is equivalent to scid -xeco -xspell -xtb.
Additionally, a filter file (.sso) can be used on the command line.