Hi all, sorry for late response.
tony59, the problem is in table styling. My plugin does not support table styles as of yet. However, I will include the fix to work around that and not to fail into the next release.
firefoxvn, please try both „cover“ settings in the plugin options. Also, you might try changing Calibre settings in the output to override DOCX output. And, lastly, image should be on the first page, first paragraph inline, followed by page break. Otherwise it is usually is not taken into account as a cover. There is a planned fix for that, but not in the next release, probably later.
Q490264, if you could attach that book, which takes so long to convert? I havent observed anything like that, and usually I am converting varying size books, 500 pages is not a limit, usually it is done really quickly: havent observed DOCX Input working longer, than few fractions of one second.
Next release is planned next Friday, 28th. I was working hard to include font embedding, there are few problems to fix yet, so haven't been looking here too often.