Hi Boxuan - thanks for the quick response!
I get a feeling that 2) might be the issue. Since JanusGraph has never allowed us to index labels, we ended up having a "shadow property" which is set as a composite index to allow us to look up by labels.

How does the data-model look like for composite indexes and the resulting rows? Does it mean that for a given "label", if we have too many vertices, the value the index will resolve too is too large?
Do you have any recommendation on how to approach this? 

Thanks,
Assaf