the latency between page load and progressive enhancement being applied shows up primarily if you load your JavaScript at the bottom of the page. Now I know that this is what Yahoo!'s performance rules suggest, I was on the team that made those recommendations. They made sense between 2006-2009, but browsers have improved significantly since then and that rule isn't necessarily true any more. You can load scripts in the head of your document without any of the blocking issues we saw in the past. You can even do multiple scripts in parallel to reduce download latency. See my <a href="http://calendar.perfplanet.com/2010/thoughts-on-performance/... on performance</a> on the 2010 performance advent calendar.
IE6 is turning into a C grade browser this quarter (http://yuiblog.com/blog/2010/11/03/gbs-update-2010q4/) so you can probably just not serve JavaScript to it (server side detection). IE7 still doesn't do parallel script downloads, so you might need to do bottom loading for it, but it's best to first find out what your users use before optimizing for it. You can get really complicated and change your page structure based on useragent and user's network characteristics, but that requires far more development effort.
