Io's an attempt to combine the language minimalism of Smalltalk and Lisp. So you get a message-passing object oriented language with Lisp-like features like programs being their own parse tree, which you can manipulate at will at runtime.
Apparently it's supposed to be really fast, too, for a pure-interpreted language, but unfortunately the name 'io' is unsearchable and I haven't found any references. (I think this is a major factor why nobody's ever heard of it.)
Io's speed has improved a lot historically, but due to its dynamic nature, it has a relatively high dispatch overhead, and little of what remains can be optimized away. In the past, GC was a big bottleneck, not sure where we are today on that front.
Io is no speed deamon, but for heavy computation, its easy to dip into compiled C or C++ libs, putting it in the same ballpark as an unjitted ruby, python, or lua.
Apparently it's supposed to be really fast, too, for a pure-interpreted language, but unfortunately the name 'io' is unsearchable and I haven't found any references. (I think this is a major factor why nobody's ever heard of it.)