Or you use the web technologies for a "native" app and focus only on the browser for that platform. Then port to others as needed, rather than trying to support them "all" from day one on a web app.

Certainly a web app is usually still not as fast and responsive as a native app, but its getting much better.

People used to say the same thing about web mail services - that they were not nearly as 'nice' as a native mail app. Now go look at how many people use Gmail and dont bother with a native mail client on their PC anymore.