I don't know.... I'm carefully watching the speed of the bouncing curser and it seems legit, as well as not any real noticeable break in video (unless this guy's a really good video mixer)
Slack would be the most believable to cut out a lot of the bs which occurs during boot but.... I would need to see a much better quality video to make a better assessment.
It would also involve a lot more than just KDE.... there are so many other variables that could contribute to the loading process.
I wish when people upload these clips they would also provide all the related info such as the specs of the machine, the hard drive set up, where and how their files are managed, etc.
This doesn't do much to boast KDE, because it is not the only factor. It's not going to work that fast for everyone, even if they use the same distro as him...(this is giving that this clip is not a trick)