As covered by Google, an excessive DOM (Document Object Model AKA web page) can harm your web page performance. It is recommended that your web page have no more than 1500 nodes, be no more than 32 nested levels deep, or have any parent node that has more than 60 child nodes.
Without completely redesigning your web page from scratch, you cannot resolve this warning. Understand that this warning is significant and if you get it for more than one or two pages in your site, you should consider:
- reducing the amount of widgets / sections within your web pages or page layouts
- using a simpler web page builder as many page builders add a lot of code bloat
- using a different theme
- using a different slider