Nova uses less RAM than Touchwiz. That is why.Fact. Even iOS lags some times. Lag is a very flexible perception, and whether or not it is really there depends a lot on what the device is doing more so than the device itself.
I have a GS4 and there is absolutely lag here and there. It's not enough to bother me though, and I find it lags much less with Nova installed. That makes little sense to me, but it seems to be the case.
Every single OS I've ever used, mobile or desktop, has lagged at some point.
And while I love my SIII, I would never keep TW or TW based ROMs on it. Running 4.2.2 custom AOKP ROM makes the SIII a joy to use. I can only imagine how fast the Nexus S4 will be on stock Android!
Don't get me wrong, there are some cool features on TW, but that small gain is surpassed by the sheer bloat of Touchwiz as a whole! Dump Touchwiz as soon as possible.