Yes, real compilers do handle `((lambda (x) ...) ...)` as well as a `let`, because as a Lisp programmer you want to be able to define and use new macros freely, without having to worry too much about low-level optimizations. Kent Dybvig gave a talk on this sort of thing called the "macro-writer's Bill of Rights". It's a similar concept to the recent talk about what optimizations need to be guaranteed for "zero-cost abstraction" in Rust and C++.