What has most impressed me about GraalVM and Truffle is their capability of deep-optimizing high-level code like Python and Ruby.

I once saw a demo where someone took a simple operation in Ruby using inefficient-but-elegant syntax (including creating and sorting an array, where a simple loop would have been the appropriate approach in C). He compiled that using TruffleRuby and the entire array construction and sorting was completely optimized out of the generated bytecode.