It's CPU intensive because of the way it's done, not because it intrinsically requires much CPU. That was my point. And it's not just algorithms; everything goes through a million layers of abstraction.
>Note that GMail is not an example of an application that really does intensive JS. A photo editing app would be a better example.
Again, I was doing photo-editing years ago with no troubles. You wouldn't even be able to start your JS photo editing app on a 1996 computer. And by focusing on "CPU-intensive" tasks you're missing an essential point, which is that stuff that shouldn't require any CPU does. My system shouldn't pause - ever. We have optimised everything for high throughput on powerful machines. The JVM suffers from exactly this problem. Java is plenty "fast" if you ignore latency.
There's just nothing demanding enough to require that going on. And yet the UI locks up for 1-2 seconds pretty frequently on my $1000 desktop machine with loads of RAM and CPU. It's even worse on my $400 laptop. There's nothing intrinsic in the hardware or the tasks that should cause this to happen - it's the design of the software. It's because the system is too complex and preemptable that this happens.