Many words have been said about the importance of performance when working with javascript files. Here I want to summarize what developers can do to increase the performance related to javascript. I found many tips on blogs. Here comes my aggregated list of actions one can do to speed up sharepoint (and not only) sites with focus on javascript:

1. Load only sharepoint stuff you need

Use prefetch option to only load sharepoint javascript files that needed. To see the difference, just add ?prefetch=0 to your sharepoint url in the browser. The downside of this lazy loading is that you wrap all sharepoint related javascript into ExecuteOrDelayUntilScriptLoaded otherwise you maybe invoke some of javascript objects and functions that are not loaded. So the prize of this huge performance improvement is a high awareness by a developer when and which scripts are loaded and run.

There is a very good wiki page which describes how to set up the environment. If you think something is missing there, just leave a comment here. Much of that was inspired by a useful tutorial from doityourselfandroid.com