Earlier quoted context omitted.
And a modern compiler will jmp past the type checks if the inferencer OKs it!
Which the inferencer probably can't do because of how dynamic standard-class can be. Also, if we want to get pedantic, method-dispatch does not dispatch on types in Common Lisp, but rather via EQL or the class of the argument. Since sub-classes can be made (and methods added or even removed) after a method invocation is compiled, there is no feasible way in a typical lisp implementation[1] to do compile-time dispatch…
To clarify, as you know CLOS isn’t the CL type system. I was particularly thinking about things like fixnum arithmetic.
The CMUCL and SBCL manual sections on code generation discuss this and it can be verified with DISASSEMBLE.