Performance overview

[robingustafsson]

We should do a performance overview of k6 (profiling), build up a better understanding of how CPU and memory is used across the k6 codebase. See if there're any low-hanging fruit and learn where more substantial work might be needed to improve resource utilization.

Based on the learnings of the performance overview we should create new issues for fixing issues we find.

To make this the most fruitful we start by creating/collecting scenarios we want to profile.

Related to #477

[luizbafilho]

I did some performance analysis and find out that most of the slowness and high CPU usage is due the standard http library. And there is very little we can do to improve its performance without rewriting it.

Although we can definitely refactor the http.request method, in order to get some small gains. For that there is already a issue #574

Here is the analisys: https://gist.github.com/luizbafilho/bcdf3b9c3aa000f5e33131657a344d2a