These document sets are the carts created for the anonymous visitors. Obviously the strategy is to be revisited because it creates a bottleneck in the index.
Mica automatically removes the document sets that have not been updated for a while. See the "Cart Life Span" setting in the General administration page. By default the value is 30 days. The clean up is triggered every day at midnight. Maybe the life span for the anonymous carts should be much shorter.
Mica decorates each indexed variable with the document set ID that refers to it. The empty carts should be skipped.
Only create a document set for the anonymous user when it needs it.
These document sets are the carts created for the anonymous visitors. Obviously the strategy is to be revisited because it creates a bottleneck in the index.