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.

I agree that the lag time is a bit higher on Z-Wave, partially because of the slower transmission speed. Plus does help that a bit. The number of hops can affect it as well, though they are limited in Z-Wave vs Zigbee which allows for more.

In a real situation this usually doesn't cause issues. Most of the zigbee devices that you're gonna have (such as light bulbs) are also acting like a zigbee router and it creates a mesh network.

You can even visualise the network if you're using zigbee2mqtt with zigbee2mqtt/bridge/networkmap

That is true provided that you have devices that correctly act as routers. Sometimes it requires extra repeaters to make the network function correctly as well. Then there is the interference issue from WIFI and Bluetooth devices.

Most zigbee light bulbs are terrible routers. Just because it's supposed to work doesn't mean it does.

Even bigger problem with 2.4 GHz is that the frequency is oversaturated and most Zigbee channels at least partially overlap with WiFi channels. I had pretty bad interference in my home before I moved my Zigbee and 2.4G wifi channels as far apart as possible.

Zigbee 3.0 will use the guard channels if there are collisions and allowed to auto select channels.

Zigbee can use sub-GHz as well.

Oh, I did not realize that. Are there any implementations that actually use that?

The UK, national smart meter effort. Sub gig, supported by three silicon providers did Zigbee sub-gig for the last mile, tough to penetrate homes and buildings. Running 3.0 on which means it will port very easily but sub-gig is much more regulated country by country than 2.4. You can search. This article covers the initiative but doesn't call out the sub-gig detail: https://telecoms.com/opinion/467631zigbee-and-the-smart-mete...