The claim is "fast as C", so I was surprised that the performance comparison was with Ruby, not with C. On my machine, the Ruby Fibonacci program executes in 47.5s, while a corresponding C program executes in 0.88s - that's a factor 54 difference, while the article reports a factor 35 for Crystal. That's good, but what causes the difference? This benchmark is pretty much all function call overhead, so I doubt it's representative of real performance-sensitive code.
The Crystal website itself makes a more modest claim than "fast as C" under its language goals: "Compile to efficient native code", which it clearly does.
All of these "fast as C" claims about modern, high-level Python-like languages (be they statically typed and natively compiled) are missing the point. It is mostly the minimalistic and terse programming style that C encourages that makes C programs performant. You avoid allocations wherever possible, you write your own custom allocators and memory pools for frequently allocated objects, you avoid copying stuff as much as possible. You craft your own data structures suited for the problem at hand, rather than using the standard "one size fits all" ones. Compare that to the "new this, new that" style of programming that's prevalent today.
I am using it for years already, and it is really performant, somewhere between C and Rust. I am still wondering why so few people use it.
Benchmark: https://github.com/kostya/benchmarks
Nim vs Rust: http://arthurtw.github.io/2015/01/12/quick-comparison-nim-vs...
Performance discussion: http://forum.nim-lang.org/t/2261
Embedded Nim: https://hookrace.net/blog/nim-binary-size/
Nim on LLVM: https://github.com/arnetheduck/nlvm