That's not totally true. A splash screen doesn't have to be used only with a program that's large or takes a while to load.
I didn't say it did have to, I am just saying that it is the done thing only to include one with large programmes. mIRC starts like a bullet, perhaps as fast as any splash would display. Whereas a programme like Klient that takes a little longer has a splash screen with a tacky looking progress bar which I think is more "all show" than anything else. Some scripters like to include a splash screen to make their script look l337. Some even include a progress bar which resembles absolutely nothing and the whole process achieves one thing - it slows mIRC down.
A splash screen is handy for apps like Photoshop that load many modules during the boot process. Bear in mind the difference between mIRC and Photoshop. Like say around the 15MB mark.
Neophyte: The "About" dialogue in mIRC is not a splash screen as it appears only after mIRC starts and not before.