> Yes, you have webworkers for multi threaded development. They're basically independent applications which run on different threads and you pass messages (which are simply objects) between them. The browsers themselves are also moving their layout and rendering engines to be multithreaded.
Yes I'm actually referring to this. The programming model. Workers are great if you can divide and conquer the problem and offload (exactly what you have mentioned). But the messaging payload would be high under some circumstances when you have to repetitively copy duplicate a lot of data to start a worker. I don't have hands-on experience with web workers but I think it is unlikely to solve the messaging overhead without introducing channels/threads. Workers are more like processes. And currently they don't have Copy-On-Write. Of course we may see improvements over time, but this is to gradually reinvent all the possible wheels from an operating system, in order to be as performant as an OS.
> A well designed app would do very little on the UI thread
I partially agree. It may do little, but in turn, the consequence may be huge. This is because DOM is not a zero-cost abstraction of a UI. It does not understand what the programmer really want to do if, say, he/she constantly ramping the transparency of a 1px div. Too much happens before the cursor blink is reflected onto a framebuffer, compared to a "native" GUI program. I think it will be very helpful if the DOM can be declarative as in XAML, where you can really say without translating them eventually into barebone bits. Developers are paying too much (the consequence) to customize this representation.
> Whatever the speedup would be the speedup the users would likely not notice or will only notice a slight improvement.
There won't be a faster-than-light word processor but I really want it to:
1. Start immediately (say 10ms instead of 1000ms) when I call it up
2. Response immediately when I interact (say 1ms instead of 100ms)
3. Reduce visual distractions until we get full 120fps. Don't do animations if we don't have 120fps.
4. If the above requirements can always be satisfied by upgrading to a better computer.
The speedup will guarantee 4) and make the performance scalable. But currently the web apps lag no matter I use a cellphone or a flagship workstation. This clearly indicates that the performance of a web app does not scale linearly with computation power, and this is not about how much javascript is executed (that part will scale I believe).