The Cost of JavaScript in 2019

User interaction can be delayed if the browser’s main thread is busy executing JavaScript, so optimizing bottlenecks with script execution time and network can be impactful. Previous versions of Chrome supported streaming parsing and compilation where the script source data coming in from the network had to make its way to Chrome’s main thread before it would be forwarded to the streamer. This often resulted in the streaming parser waiting for data that arrived from the network already, but had not yet been forwarded to the streaming task as it was blocked by other work on the main thread (like HTML parsing, layout, or JavaScript execution).

Source: v8.dev