The design of web APIs means that a lot of stuff has to happen as if it were on the main UI thread, so while you can do fork/join stuff to optimize it - for exa

The design of web APIs means that a lot of stuff has to happen as if it were on the main UI thread, so while you can do fork/join stuff to optimize it - for example, doing layout in parallel, etc - you're still bottlenecked by how fast the main thread is able to do all the stuff it needs to do. This is why many APIs disallow synchronous use or have serious constraints on synchronous use now.