At least in the part of Google where I work (Technical Infrastructure), in general most of the obvious optimizations that can be made at only one level (within

At least in the part of Google where I work (Technical Infrastructure), in general most of the obvious optimizations that can be made at only one level (within the scope of a single programmer, or a single team), have been made long ago. So most of the changes to make the system more efficient will require coordinated changes across multiple teams, and multiple pieces of software, and in some cases, may impact more than one SRE team. In that kind of situation, you betcha we need to have design docs! And in terms of making it easy for other developers to shoot down ideas, very often they may know about some dependency or key assumption in some other piece of code that you didn't know about it. And it's better to find out about it during the design phase, than to have to rework 50% of your work when you find out about it at code review time, or worse, if it gets deployed and you get angry notes from SRE's that were woken up at 3am and you need to send them a bottle of whiskey to apologize for your f*ck up....