Agreed -- however, you can download via http://keepvid.com and then watch in VLC, where you can use the "]" and "[" keys to go faster/slower by 10% increments.
Actually I don't know whether KeepVid will download a Dtube video; if not, they should soon! :)
keepvid says invalid url; so does http://www.clipconverter.cc and neither does 4k video downloader work
Well that's too bad! Hopefully enough of us report the issue and they fix it -- on whichever platform (i.e., add speed control to Dtube, and/or add downloading of Dtube videos to Keepvid and clipconverter).