> what prevents US AI labs from doing the same level of software optimization?

Because they don’t have to. Most of the time money would buy you newest and/or more hardwares so there’s low/minimal interest to optimize the code or approach.

I would be extremely surprised if US labs weren't aggressively trying to optimise their stacks in exactly the same way. Any gain in performance or efficiency directly affects the bottom line as well as research speed.

They are making these optimizations. They publish these reports too if you care to look.