What is the fastest way to convert a large volume of documents (.html) to drupal nodes
The module that appears as most suitable for you is called HTML import. This module will divide one single large HTML document into a structured Drupal book where the heading level hierarchy is respected. This module works with HTML exported from Word; HTML document converted from PDF as well as HTML document exported from Adobe InDesign. 

This makes it possible to use a single HTML page, created in a word processing program and saved as HTML, or other structured HTML, to create a multipage Drupal book in a single step.  You actually need to collate the output from all your .docx files into a single HTML-file before converting to HTML to use HTML import in a single step.

In case your requirements are beyond what HTML import is capable of, below is a list of all the (usable) Drupal modules I am aware of that can be used to bulk import HTML (and some other formats) into a Drupal site:


HTML import  - Import a single HTML page created from MS Word and split it up into an hierarchical Drupal book structure consisting of a set of interlinked Drupal nodes.
HTML2Book - This also imports a single HTML page and split it up into an hierarchical Drupal book structure consisting of a set of interlinked Drupal nodes, but requires you to clean up the tagsoup created by MS Word first.
Import HTML - Import all of an existing, static HTML-site into a Drupal site as nodes.
Feeds - Import or aggregate data as nodes, users, taxonomy terms or simple database records.
Migrate - Provides a flexible framework for migrating content into Drupal from other sources, including HTML.
Node import - Allows users to import content (node, user, taxonomy) from CSV or TSV files. Requires you to convert from HTML to CSV. (no Drupal 7 version)


Of these, the simplest to use are HTML import and HTML2Book. However, both can only handle a single HTML page. For more complex conversion tasks, where you need to convert a large static HTML-site made up of several interlinked pages, one of the other modules may be more suitable.  

Also: Whatever tool you use, never experiment with bulk import on a production site. Unless you're a lot more clever than me, there are going to be false starts and botched imports. Cleaning up thousands of nodes damaged by an import gone wrong is not fun.  Always experiment with bulk import on a throwaway staging site that just can be discarded wholesale when things go wrong. Transfer the settings and do the import on the production site when you're sure you have a working set-up.