You may already know this, but let-bindings are not necessarily on the stack. The reference does say they are (it's important to remember that the reference is not normative), and it is often simpler to think of them that way, but in reality they don't have to be on the stack.

The compiler can perform all sorts of optimizations, and on most modern CPU architectures, it is better to shove as many values into registers as possible. If you don't take the address of a variable, you don't run out of registers, and you don't call other, non-inlined functions, then let-bindings (and function arguments/return values) need not ever spill onto the stack.

In some cases, values don't even get into registers. Small numeric constants (literals, consts, immutable lets) can simply be inlined as immediate values in the assembly/machine code. In the other direction, large constant arrays and strings don't spill onto the stack but rather the constant pool.