The browser does this already.
> Even if your payload is very small, the HTTP round trip can make the interaction feel slow, especially if your browser has to go through the full load+render routine. Things like images and sometimes even the whole page will flicker.
Browsers are very smart these days, caching many things. And if the bloat is eliminated (unnecessarily large images [make thumbnails], custom widgets [use what's there], etc.), if you webpage is not the whole of the Emacs or FreeBSD manual, it'll load in about a second or so at worst. The progressbar will indicate that it's loading, so the user will know that the server is live and he's not standing there waiting to see a HTTP 50* or 40* or a lookup error.
> Say you have a page with a few interactive buttons/links and a form with text input. Without any AJAX, if you click a button or link (that isn't the form's submit button), it will wipe anything you had entered into the text field. Not a great experience.
Also an inexistent experience. Most browsers do retain the contents after navigation, both ways. I use xombrero and it does. Chrome too, I just tried it.
If downloads of your web pages are >1sec, check your web application, web server, proxies, asset file sizes, loading order, internet connection, status of hosting, etc. Use default widgets on your forms and don't fiddle with their default behaviour. Otherwise when everybody puts out their shiny new idea, the users become timid to click anything. But I guess if all the websites were like this thousands of front-end devs and web designers would lose their jobs, and thus we have all these websites.