A large DOM tree can slow down your page performance in multiple ways:
Sitefig reports the total DOM elements for a page, the page's maximum DOM depth, and its maximum child elements:
Sitefig flags pages with DOM trees that:
In general, look for ways to create DOM nodes only when needed, and destroy nodes when they're no longer needed.
If you're currently shipping a large DOM tree, try loading your page and manually noting which nodes are displayed. Perhaps you can remove the undisplayed nodes from the initially loaded document and only create them after a relevant user interaction, such as a scroll or a button click.
If you create DOM nodes at runtime, Subtree Modification DOM Change Breakpoints can help you pinpoint when nodes get created.
If you can't avoid a large DOM tree, another approach for improving rendering performance is simplifying your CSS selectors. See Google's Reduce the Scope and Complexity of Style Calculations for more information.