Only if you are writing small things. Dynamically typed languages are often called "scripting languages". They are productive for scripts. Once programs get large though, the lack of static type and syntax checking generally slows things down. Refactoring dynamically typed code can be an absolute nightmare because nothing in the language is helping to make sure that your change doesn't cause problems in other parts of the code.

When you start a new project, don't just think about what will be fastest to write the first 100-1000 lines in. Think about the long term. If you have big goals for that project, start off in a language that can support those goals, but in terms of speed of execution and technical issues when things get big. Starting a project is one of these languages is a great way to have to rewrite your system if you actually become successful.