The problem is that as soon as there is an update all admin pages get that red notification on top. To load that it has to do some cloud calls to see how many updates are ready for you, that takes time. In case of no updates the call to the cloud takes less time and no notification is shown.
So to have a correct measure of possible slowness you could test for http first byte, or with @bptworld hub watchdog. [RELEASE] Hub Watchdog - Simple way to monitor if your hub is slowing down or not
