I don't like splash screen for the purpose of a splash screen (like ESET do) but I feel your pain if you have a slow computer and you don't know if Inkscape is actually starting up or not. The best solution I've seen to this problem is the one GIMP uses. They have a splash screen _with_ a loading bar for all modules and settings that are loaded during start up.
Just as Martin Owens says, something needs to be shown to the user in about 500ms otherwise another instance is started with another double-click on the icon.
Martin, what do you mean with a "build flag for a splash screen"? a loading bar just like GIMP?