You don't need multithreading to get concurrent asset streaming, a completion callback or async-await-style code will work too (after all, that's how most Javascript web games load their assets "in the background"). Also, browsers typically restrict concurrent download streams to about 6 (the exact number is entirely up to the browser though) - so you can have at most 6 asset files 'in flight'. In the end you are still limited by the user's internet bandwidth of course.