How many associations do you have set? If configured wrong those could possibly be causing issues.
Also, next time when it gets bad before you reboot refresh the zwave stats. Look for anything with a high RTT or a high number of route changes. Also a high PER count could be a clue.
One problematic device can bog the whole thing down and grind it to a hault.
The third party app zwave mesh tool can also show the number of messages and lets you sort by anything to find the worst devices [BETA] A Z-Wave Mesh Tool [C7 and 2.2.4+ Only]
