I have about 20 Z-Wave switches in my home, spread across 4 models and 2 brands. There were noticeable differences in response time, both from the physical button and over Z-Wave, when comparing brands. And even the most reliable brand for me (Aeotec) has had issues with the network being a little laggy. That's still a success in my books, though; I have bought Z-Wave devices that do not play nicely at all.

On the other hand, my Philips Hue lights have all been absolutely flawless. They fire on time, every time.

I think implementation of the standards matters a lot more than the standard itself, in practice.