I worked at EA for eight years and have worked at Google for the past ten. Google and the GMail team most certainly understand how to write performance critical code. Your favorite game would take 10 seconds to load and run at 2FPS too if it had to be pushed over the Internet every time it started up and run inside any of a few only mostly-compatible VMs for a dynamically-typed scripting language never designed for anything more important than making buttons light up when you hover over them.

You're correct that optimized code is generally harder to maintain. Optimization often requires punching through abstraction layers or calcifying certain constraints or assumptions in the code.