Flat design is suppose to be more efficient, however based on our evolutionary history, an all out flat design actually increases the effort the brain needs to go through to process symbols. But really, this achieved efficiency doesn't matter as the difference between flat and adding some shading is negligible. What matters is whether or not we perceive the design as attractive, and that is a purely cultural and subjective factor.
Flat design does have efficiency improvements but it's not where you think it is. The efficiency improvement is not on the user side, but for the developer. Anyone with the talent to make a cover photo, draw a flat square and paste some text on it can now call themselves a professional designer.
Zero talent + minimal effort = Clean Minimal Design.
Maybe I'm exaggerating on the amount of effort required but definitely you don't need the ability to paint like leonardo in order to make a minimal layout.