First of all, I would like to thank @codahq for bringing Zniffering to our attention, and for allowing us (really for the first time) to "see" our zwave networks. If you have an extensive zwave network, and you're having issues, please see his post which I've inserted at the bottom of this post.
I'm writing this post to help other Hubitaters who may read this at some point in time in the future.
- A few (3-5) years ago, when I first got started in Home Automation, and I was convinced that z-wave was the answer (days of Vera, HomeAssistant, etc.), I thought that it was a good idea to build out my zwave network to the furthest reaches of my house. I reasoned that no matter where I would put my zwave devices (especially un-powered sensors), I would be covered by a good mesh. So, I purchased quite a few zwave outlets and scattered them around my home. Zniffering allowed me to see that my newer zwave plus devices had such a great range that I didn't need any of those zwave outlets. Furthermore, some of those outlets were actually slowing down my zwave communications. In a burst of energy (aided by a forced isolation), I replaced 6 of them in one day. I must have run at least 3 (maybe 4) zwave repairs that day.
- Lo and behold - I have a small zwave simple lighting automation, which I run to see how my zwave is doing. I count the number of seconds for it to execute, which is usually 1-2 seconds. The day that I did all of those zwave repairs, that automation took 17 seconds. Hmmm... I had just rebuilt the mesh tables with a zwave repair - why did it take so long? Could it be that each zwave repair exhausts some part of some resource, so that after a number of zwave repairs everything runs slow? I really can't say for sure, but I can say that next day after a reboot, that same automation ran in under 1 second.
Unfortunately, I don't know why we have the "Slowdown" disease. I know that if you have it, unfortunately you have to investigate and investigate, and investigate. Maybe you will eventually find the right antibody.