I'm a WP user, though I don't use the YouTube app much. To be honest, this really blows.
Based on the links on this thread, I think Google is the one more likely at fault. Double standards, much?
If you disable network discovery, this means that you also have to disable file and folder sharing.
The best you could do is disable all public folders in WHS and just password protect them or limit them to certain user accounts.
It's quite surprising that you have 75% of free disk space, considering Out of the Box units have lesser, plus you have the 8.1 update. Did you delete your Windows Restore Data?
As for the media lagging, I remember Linus published the same findings. VLC might release a Metro Modern UI soon.