<p>Our web application renders fast in some IE browsers, slow in others...  It seems to be an HTML rendering problem...  The first 10% of the page displays immediately, the last 90% takes up to 10 seconds, and this is static content.  I've run with firefox/yslow, renders very quickly.  Seems to be isolated to some users/configurations of ie.  Quirks mode does not seem to make a difference.</p>

<p>Is there a tool or application that I can use to help me discover a rendering bottleneck?  Am I doing something egregious in my code?  Could it be a javascript issue?  Any help or suggestions will be much appreciated.  thanks.</p>

Use Fiddler to look at the times to load images, css, js files, etc.  In other words, is caching a problem?  Javascript can definitely cause issues in different browser versions.  There's lots of optimizations you find in some versions that aren't in others.  Also, make sure your html is well-formed xhtml if possible.  How the page is arranged can also affect life.  If your document tree is deep, it may need to wait to render large sections until it reads all the child nodes.  Another thing to note, certain toolbars and plugins do look ahead loading and can slow down life.   An HTTP Proxy can help you watch what's going on network-wise at least.
Not sure if anything of those ideas might help your exact problem, but they can help life overall.