DTube is a different application than Steemit with a different team of developers, both apps connect to the same blockchain (STEEM) but they only share the text content not images or video. However, DTube is also Beta software and you are correct in pointing out that sometimes there are buffering issues for the reasons that @schrosct mentioned (which has more to do with the connection to the IPFS nodes to your local computer than with the DTube software).