How do you do this exactly? Because if you are just calling the http time to load the devices page I can easily explain why you experience this.
Btw, I too have a suspicion that the update notification can impact response time of the hub, but haven't noticed it with all updates. Only two times, and the last three updates I didn't anymore. So I expected they've solved it. There is another topic or metion about this from a couple of month ago if I remember correctly.
Edit: found it Slow Hub when there is an update available
