Really, what he's saying is that it saves a lot of time to prototype code in a convenient language, and then translate parts to optimized code in a fast languag

Really, what he's saying is that it saves a lot of time to prototype code in a convenient language, and then translate parts to optimized code in a fast language (where necessary), once good algorithms have been worked out. Many fast languages get their speed by trading away flexibility. This is not specific to Scheme, though - while it's a good prototyping language, so are Lua (my favorite, and LuaJIT reduces the need for C), awk, Python, etc. Knowing how to implement the prototyping languages' constructs efficiently in the "fast" language is important, though. Of course, "fast" and "convenient" can also be the same language, such as prototyping in Common Lisp and then adding declarations. OCaml is also quite good.