Unlambda [1] is completely functional and has only 2 built-in functions. According to the author, it's the perfect programming language, even though it's extremely inefficient and the code is almost impossible to understand. Many of the features he lists enhance either efficiency (number types) or readability. The languages that lack these things are bound to be slow and hard to understand at least in some cases.

[1] https://en.wikipedia.org/wiki/Unlambda