My opinion is that JavaScript applications aren't losing that much performance by the use of lodash or fast.js but the gain in code brevity and readability more

My opinion is that JavaScript applications aren't losing that much performance by the use of lodash or fast.js but the gain in code brevity and readability more than makes up for any slight performance reduction. (Especially in a large web app where the tiny lodash library can save you many KB of boilerplate). And with regard to performance, for most Node.js code / webapps if you have a loop with so many iterations that _.map is significantly slower than a for loop then you might be doing something wrong.