Cesium uses Web Workers to create geometry in the background so that the rendering thread is not stalled and performance drops. Cesium optimizes Web Workers so that they are not running constantly, and they shouldn’t be spinning up unless new geometry is being created repeatedly.
Would you be able to share a little bit more information about when you are seeing this? Does it happen in all situations, or with a particular dataset? It would be useful to know about the data/geometry in the scene to understand whether new geometry is being created repeatedly. As always, anything you can share in a sandcastle will help, but we can understand if this is more integrated into your application.