What's the difference between 'json" gem and 'json_pure' gem?
Nokogiri needs extra libraries because it uses libXML. json has no library dependencies, so you don't need anything extra.
The advantage of json is what you said: It is in C, so performance is better than json_pure.
The advantage of json_pure can be if you don't have MRI. For example, the json_pure gem can be installed in JRuby/Maglev or other implementations. The json gem can't be; it needs an FFI plugin to do that.