To all the people saying "it's dangerous to add concurrency to javascript", javascript has already workers, shared array buffers and atomics. It's entirely possible today to start two or more workers, pass a shared array buffer via a message and then write concurrently on the same buffer forfeiting message passing and synchronizing only using atomics. You can even do lock less data structures, see for example https://greenvitriol.com/posts/lockless-allocator. That's what you do when you write high performance Web apps. This proposal only adds lightweight threads sharing memory by default, but it's by no means the first and only way to do low level concurrency with javascript.
This is from PR: v1 collects synchronous and stop-the-world
Go build performant web app that has stop-the-world all the time.
You bypass GC by allocating pools of array buffers at the cost of low-level memory management.
So you add threads to allow a shared objects graph with a side effect of more frequent stop the world. Then you fight stop the world by resorting to low level memory management. But if you go level why not allocate a shared array buffer and share it across workers? Something you can do today without threads.