Zwave is designed for low traffic, tiny message networks of up to 250 devices. The assumption is that most of the devices sleep most of the time, or at least are inactive from a network messaging standpoint. In fact this is exactly why the concept of message relaying works: most devices aren’t busy, so they can happily pass on messages for other devices. (Only mains-powered devices act as repeaters, though, not battery-powered devices.)
If you have a network of nothing but battery powered devices then, yes, things might slow down as you go over a dozen or so as the hub itself might get overwhelmed. A more typical mix would be one repeating device for every 5 battery operated devices if do, and then the mesh would actually get stronger as more devices are added and more alternate routing paths become available.
Then there’s the traffic issue. Zwave is not intended for real time monitoring, but rather sampling. A typical zwave motion sensor, for example, likely is set by the factory to sleep most of the time, and wake up every 4 or 8 minutes to see if anything is going on. Then if the delta change reaches their reporting threshhold, they send the report and go back to sleep. Low traffic, tiny messages.
If instead you crank the sensitivity up very high, or you shorten the sleep interval, or you poll the devices all the time, you can vastly increase the amount of traffic on the network. Then anything might slow down unpredictably.
And finally there’s the range issue. If a device is almost but not quite out of range then it can have a hard time getting messages through. And again, not enough repeaters can make the problem worse.
So there will be some homes with 200 zwave devices, low traffic, repeaters in the right places, and everything ticking along beautifully.
And there will be other homes with 7 devices, high traffic, no repeaters, and messages getting delayed or lost.
“All home automation is local” 
Support may be able to help diagnose signal strength or traffic if you’re concerned about network performance.