DOM traversal does tend to be slow in IE7.
There is also a series of articles on the IEBlog about performance optimization, they are a few years old, but still relevant.
The article discusses some of the obvious optimizations such as variable caching and using local function pointer variables, but one of the more interesting methods is the caching of DOM accessors.
e.g. using document.getElementById("someID"); in a for loop would be quite bad, so you should cache it whenever possible.
Unfortunately the article also describes some (surprise surprise) IE only methods of caching DOM methods, like "appendChild()".
This will throw an error in non-IE browsers. I probably wouldn't recommend using DOM method caching anyway after I just did a quick test I noticed that simply caching the DOM element accessor (i.e. getElementById) was much quicker in IE7/8, Fx and Chrome.
Hope this helped a little bit.
Oh, also, are you applying event handlers to table rows/cells?