Remember that zigbee is subject to interference from Wi-Fi. One reason it is not as popular for locks is that if you are trying to boost Wi-Fi around your door in order to support Wi-Fi cameras or video doorbells, you are quite likely to interfere with the lock’s operations. I’m not saying it’s impossible to use a zigbee lock successfully, but given that smartthings gives us no network diagnostics, it can be very tricky to troubleshoot if you do run into problems.
The beaming used for zwave locks does run slightly slower than regular Z wave, but it shouldn’t be noticeable to most people if you have a good mesh.
I would not say “secure devices slow down your zwave mesh” because
a) that’s not what is happening and
B) it’s going to confuse people about the new S2 security protocol, which is in fact faster than older generations. For example, I haven’t seen any complaints about the new ring protect alarm system, and it’s all S2 zwave.