"Fast as C" should maybe be changed to something else. Crystal can bind directly to C functions and libraries without any overhead, plus you have pointers and other stuff. So you could really write code that runs as fast as C if you program like in C, but with Crystal syntax. That's where the tagline comes from. But of with course normal usage of the language, with its heap-allocated stuff, garbage collector and usual stuff (like Array#map, which does allocate a new array), you won't get performance like C, but it will be acceptable, and usually faster than dynamic/interpreted languages.
That's exactly what the article I linked said, isn't it? It has low pause times, at the expense of everything else.
Beware trading a visible cost for an unbounded number of smaller costs. In Go's case, in return for minimal pausing you get memory fragmentation, terrible performance if the GC is ever seriously taxed, and a 100% (!) heap memory use overhead. If you're just writing commandline daemons or servers you restart every ten minutes though, who cares?
Go doesn't come off too badly in this because it's not Java, which allocates like crazy. That's a laudable tradeoff, but that does not mean they solved garbage collection very well.
What other languages? Because the language uses "Fast as C" as part of it's tagline. Is this a false claim?