> The other common proposition is that dynamically typed languages are faster to write in than statically typed languages.

This part is also very tricky, as most of people's hard-earned experience with this is (almost by definition) old. In recent years, type inference has reduced the type-caused slowdown immensely. E.g. in Haskell you can (+) write large statically typed programs without specifying any types at all - they are inferred by the compiler.

(+) But please don't.