I think my random delays have been solved with the excellent help of the support team and some exploration of my wi-fi settings. We identified it was the zigbee sensors that were the issue, everything z-wave seemed to work well. I’d tried moving the hub away from the router - extending the short cable that comes with the hub so that it would be 10 feet away, but this seemed to have only a short term effect. I hadn’t realized that zigbee is sensitive to wifi interference nor that the hub has a preset channel that doesn’t change. My router was set to choose channel automatically - and using a wifi analyser I found it would change channels from time to time. Some of these overlap the range of the zigbee channel (the numbers don’t match but there are some clear pictures out there that show this well), see link below. Setting my router to a single channel for 2.4GHz connection as far away from the zigbee channel frequency had an instant effect! Now all the automations work instantly - in fact blindingly quickly and the whole system seems so much more robust.
So my thanks to support for working this through, a request to have this emphasized more in the documentation and a reference to a really useful discussion that I hadn’t been taking notice of until I started exploring the issue. FAQ: Networking and Reducing Channel interference between WiFi and Zigbee