The FAANGS certainly have people doing this kind of work. For example, Andrei Alexandrescu has a bunch of keynote talks at CPPCon where he talks about 1-4% speed optimisations he makes to the std library at Facebook.

https://youtu.be/FJJTYQYB1JQ

I also like the talk by Nicholas Ormrod “The strange details of std::string at Facebook":

https://youtu.be/kPR8h4-qZdk