Thanks James!
The only thing I’d add here is I think there may be a special case for labels, where CesiumJS will hold onto the memory created because in the general case it’s a lot faster than destroying and recreating those resources.
After some investigation, I think this is what you’re running into:
For labels, one billboard is used per character to visualize the label. Once the labels are removed, we keep these billboards around to be reused later when you create another label, because reusing a billboard has better performance than always creating new ones. However, I do agree it doesn’t make sense to keep around such a large number of these objects around indefinitely.