Using GroupDocs.Viewer 22.1.1 to convert the attached DOCX file to HTML takes approximately 16 seconds in .NET 4.7.2.
Could you please check why conversion takes so long for just 18 pages?
using (var viewer = new Viewer(doc))
{
var htmlViewOpts = HtmlViewOptions.ForEmbeddedResources();
var viewOptions = ViewInfoOptions.FromHtmlViewOptions(htmlViewOpts);
var info = viewer.GetViewInfo(viewOptions);
var options = HtmlViewOptions.ForEmbeddedResources();
viewer.View(options);
}