Sorry but there isn't a way to switch it off right now. We will try to speed up the check so it doesn't slow you down during start up.
The Community notification comes up only when there is new information on the Community page. You don't have to visit the page when the notice comes up. The notification is provided to ensure that you don't miss out on important information, updates, and answers to support questions.
Kyma checks for new content once per day, so, for example, if you launch Kyma a second time that same day, there won't be a notification that second time. In other words, it doesn't check every time you start the application — just once per day.