Photoshop really is not that complex - you could write a photoshop clone that would load in under a second, easily. But it loads all sorts of stuff you do not need up front - many fonts, textures, drivers for scanners, cameras, and so forth. I think the real problem is that Adobe seems to keep patching an already bloated code base - I think if they started fresh they could redesign it more efficiently...just my humble opinion...
I think it would be difficult to write a photoshop clone that loads in under a second and provides the same level of performance when dealing with advanced filters on huge images with hundreds of layers.
Anyway I doubt that all of that stuff is loaded at startup anyway.
