I can see the same behaviour using Vaadin 24.5.4. The problem is in the asyncWriteService (and may be executorService too) of the BroadcasterConfig class in the Atmosphere framework.
If I force 500ms updates of the UI I can see a lot of new created threads (> 100). This will result in an unusable Web-App in the real world.
Is there a way to access the BroadcasterConfig to set own services?