Obviously, talking about speed Ruby loses. Even though benchmark tests suggest that Ruby is not so much slower than PHP. But in return, you are getting easy-to-maintain DRY code, the best out of all frameworks in various languages.
For a small project, you wont feel any slowness (I mean until like <50K users) given that no complex calculations are used in the code, just the mainstream stuff.
For a bigger project, paying for resources pays off and is cheaper than developer wages. In addition, writing code on RoR turns out to be much faster than any other.
In 2014 this magnitude of speed difference you're talking about is for most websites insignificant.
