The issue isn’t so much having additional threads, it’s needing two main threads. The browser has one main thread for accepting user input and then dispatches t

The issue isn’t so much having additional threads, it’s needing two main threads. The browser has one main thread for accepting user input and then dispatches the relevant user events to the JS main thread. The browser can either dispatch the events asynchronously, leading to the events being handled in a noticeably delayed way, or the browser can block its main thread until the JS dispatch finishes, leading to fewer UI events being handled. Either way is an inferior experience.