Today's web applications can get pretty big, especially the JavaScript part of them. As of mid-2018, HTTP Archive puts the median transfer size of JavaScript on mobile devices at approximately 350 KB. And this is just transfer size! JavaScript is often compressed when sent over the network, meaning that the actual amount of JavaScript is quite a bit more after the browser decompresses it. That's important to point out, because as far as resource processing is concerned, compression is irrelevant. 900 KB of decompressed JavaScript is still 900 KB to the parser and compiler, even though it may be roughly 300 KB when compressed.
JavaScript is an expensive resource to process. Unlike images which only incur relatively trivial decode time once downloaded, JavaScript must be parsed, compiled, and then finally executed. Byte for byte, this makes JavaScript more expensive than other types of resources.