It's not that the Z-Wave stick falls behind, when a Z-Wave device is broadcasting everything else has to go quiet (nature of RF), but the transmission speed is just very low, so when chatty devices (think sensors that report multiple bits of information such as humidity, temperature, light level, UV level, and more) are sending data you can't also turn on a light.

When you have a lot of those devices on the Z-Wave network (multiple motion sensors in a room to do more accurate presence detection and the like) those chatty devices slow down the network tremendously.