For me the update seemed to speed things up on my Vox but lately I have noticed (despite using Advanced Task Killer) that my Vox is once again slowing down. It once again takes forever to load and completely come to life when being turned on at first. It's very slow when surfing the internet and using facebook (despite good internet speeds). When it decides to update the Kobo library it takes forever (even when there are no new books for it to download or only one new book). Also, while updating the library or checking for updates it is impossible to do anything else on the Vox without it locking up. When I have tried to watch a video from YouTube on my Vox its been so slow that I usually give up before the video starts to stream. I don't really understand why after having experienced such an improvement from the update in the beginning things are now once again coming undone. Using my Vox for anything outside of eReading is now pretty much out of the question.
Good luck solving your issues with YouTube. At this point I'm done with trying to troubleshoot my issues or dealing with Kobo Support. I'll just use it as an eReader until it doesn't work for that purpose and then replace it with an android tablet and run the Kobo app there.