I actually ran into an issue recently with CPU intensive tasks blocking my web server. It turns out that "querystring" (used to parse request bodies in web applications) is an asynchronous, blocking request. You'd never notice much slowness, until your request bodies are massive (think 50 nested JSON objects and some base64 image data for good measure) and you have multiple per second. Now, every request is blocked until the previous one is processed. I'm still trying to figure out a solution, after looking into worker threads, etc.
https://www.npmjs.com/package/fast-url-parser for url parsing (the built in url parser is the main reason why node is so far behind on the TechEmpower benchmark - with this replacement the benchmark shows about 60-80% improvement in served req/s)
For very large post bodies, I use JSON in conjunction with OboeJS - http://oboejs.com/ . Its not too much slower than native JSON.parse (about 5-7 times) however its non-blocking. Still haven't found a solution that is close enough in speed to native JSON.parse
