Yes, global secondary index is better than local secondary index, from the user view.
I think App Engine also adopted the same strategy for Cloud Datastore. The performance of parallel second indexing in Cloud Datastore is incredible. I have a table with about 20 secondary indexes. Some of the indexes belong to exploding indexes. So averagely, for each new row inserted, there will about 100 index rows inserted. I never detected an obvious delay in building the indexes.
