Converting the attached PPTX files to HTML pages, results in file sizes between (20-90 MB per page).
Additionally the conversion is really slow in that case.
PPTX size problem.zip (3.6 MB)
GroupDocs.Viewer 20.8 was used with HtmlViewOptions.ForEmbeddedResources without special options.
This issue is reproduced at our end. Hence, we’ve logged it in our internal issue tracking system with ID VIEWERNET-2716. You’ll be notified in case of any update.
In order to reduce output files size, please set HtmlViewOptions.ExcludeFonts to true. By default, we’re embedding all the fonts. When embedding fonts we’re encoding them as Base64 string and as a result, the output size may increase and processing slows down. Here is the output.zip (191.9 KB) rendered with HtmlViewOptions.ExcludeFonts setting set to true. Hopefully, it’ll resolve your issue.
Yes this fixes the problem with processing speed and file size 
Thanks a lot for the quick resolution!