I've red about the asm.js project http://asmjs.org/ and that Mozilla and Chrome will support it in the near future.
Are there any plans or better asked: Is there a chance to migrate ExtJS into asm.js compatible code?
asm has the advantage that it runs on current JS-Engines if you trust the statements: http://asmjs.org/faq.html
However - ExtJS could profit from the promised speed up and would wipe out the big point of criticism that the performace is bad.
On the other hand is only the performace in old IEs the bottleneck...
What do you think about it?