There can be many different things that cause this, and you may be running into several of them, or different ones at different times. 
@Matt_Behnken already detailed several possibilities, let me just toss out a few more.
Sensor issues
One of the most common in the setup you describe is that the motion sensor, which is battery-powered, was asleep when you first walked in. (No, reallyāitās a āsleepy deviceā to conserve battery life.).
It can also have to do with motion sensor placement.
Iām not feeling very well today, so Iām not going to go into a lot of details, others can cover any follow up, but the point is just that motion sensors themselves can be tricky.
Broken mesh?
One other question: do you have any zigbee bulbs connected directly to the SmartThings hub? (Not to a hue bridge.) I understand you are trying to use the sensor to turn on a zwave switch, but I just want to know if there any bulbs that the sensor might be trying to use as a repeater.
Also, when is the last time you ran the zwave repair utility and what results did you get?
The point of these two questions is that if the sensor has multiple paths to the hub, or the hub has multiple paths to the switch, it may be that some of those paths work better than others. So it could be that if the efficient path happens to be busy at the time that you walk into the room, the message is getting routed along an alternate path that doesnāt work as well. This kind of thing is a common reason for inconsistent performance in a mesh network and in this case you are dealing with two different mesh networks.
Further Troubleshooting
If none of the suggestions brought up so far seem to help, the next step Iād do would be to try to determine where specifically in the sequence the lag had occurred. Did the sensor fire and there was a lag before the light came on? Or did the sensor fail to fire? Or did the sensor fire but the message didnāt go through?
Again, Iāll leave the details of how to do all that to other people for now.