Things have changed a lot after two decades of Microsoft browsers. If by the late 90s the company attacked Java (and JavaScript) in browsers, Microsoft is now a champion of the importance of browsers having a good JavaScript engine to speed up websites and web applications.
Microsoft Edge is the new browser that will come with Windows 10, and JavaScript performance tests now place it as the fastest engine. This has been made possible by changes in Chakra (the name of the JS engine in Edge) in the 10122 update of Windows 10 that was distributed yesterday.
The tests are Google Octane 2.0, doing better than even Chrome, and Apple Jet Stream with the same results. It outperforms Internet Explorer 11 by far, but this is actually something that has little merit today.
Microsoft has improved JavaScript performance using various techniques, such as inserting JavaScript code functions within the code of the functions that call them (not in all cases, but in those that can improve the context switching times), improvements in the use of global constants, improvements in try-catch blocks, execution of non-minified code blocks, and optimized access to certain positions of an indexOf array.
You can read more about these exciting changes in this article by Microsoft.