Yep but all these checks don't necessarily need to happen at runtime. A clever enough compiler can optimise all of it (or most of it) at compile time. In genera

Yep but all these checks don't necessarily need to happen at runtime. A clever enough compiler can optimise all of it (or most of it) at compile time. In general this is a very hard problem but for something static enough (like a simple parser) the compiler can produce code and determine its safety without needing to carry checks on to the runtime. Note that C code or machine code would also have to perform input checks at runtime when doing http header parsing. A Common Lisp compiler wouldn't theoretically need to produce code performing any more runtime checks than these.