How do you make complex designs faster?

I'd actually look into Clark Aldrichs shortsim approach. Agin - "it depends" - but the shortsim methodology he's using is super-fast and makes a real difference. Mind you - storyline is extremely ill-suited for quick scenario branching compared to other tools like iSpring talkmaster og branchtrack. It's good for complex stuff involving variables, employing videos etc. 

The "it depends" part of my answer relates to how complex you want your scenario to be. Not as in number of decisions made - that's easily tracked and colorcoded in branchtrack (which is easily embedded in Storyline by the way), but as in the complexity of the visual and the role they play on each slide. Branchtrack only supports images, so you'd have to create your visuals to match.