I don't have any lag or animation jankiness when multitasking on my Nexus 4 with CyanogenMod.yes TouchWiz is horrendously ugly, so ugly that its almost fashionable. Android lag/animation jankiness in general when multitasking is quite bothersome, not solely a tw issue.
So yes...TouchWiz, Sense, and the other bloated Android skins then are clearly the problem.