I would definitely do that Z-wave firmware update on the Z-wave page. It is different than the hub firmware. There were significant improvements to Z-wave radios with that update.
How many events are you storing for that thermostat. I would dial those way back, there really isn't a reason to store hundreds upon hundreds of thermostat events. In fact, I think it is a good practice in general to limit events unless you really need them for some odd reason. Nobody in their right mind analyzes how many times you turned on a light in a month, so these events are mostly clutter.
And yes there are LOTS of updates in the latest firmware. There are even LOTS of updates in the 2.3.4.xxx series after the 2.3.4.117 update you are on. I would advise updating.
Edit: in particular...
- Possibly fixed elevated/severe hub load experienced on previous 2.3.4 builds.
and * Fixed high CPU usage/memory leak under certain conditions.
And those are just a couple in the 2.3.4 series.
