The effect of JavaScript on mobile web performance is twofold.
One, it is the second largest contributor to webpage weight, behind images, thereby increasing download time; and two, once downloaded, the browser then needs to run the script, which can delay the downloading/rendering of other (perhaps more important) assets on the page.
JavaScript (aka scripts or JS) is one of the triumvirate of technologies that make web pages (and web apps) work. The HTML (Hypertext Markup Language) controls the structure and content of the webpage; CSS (Cascade Styling Sheets) controls how the site looks on different devices; and JavaScript makes the page more interactive and dynamic.
