With the new index: creating and deleting 10,000 documents took 32659.20 and 33152.80 milliseconds in 2 tests and the cache size was 8.3 MB.

Without the new index (the released Firestore): creating and deleting 10,000 documents took 31744.20 and 31649.80 milliseconds in 2 tests and the cache size was 7.5 and 9.7 MB.
It looks like Chrome pretty aggressively compresses the indexeddb contents in the background.
Conclusion: The slight increase in document creation time (1.5 seconds per 10,000 documents) and database size (7.5 MB -> 8.3 MB) is acceptable.

Author: dconeybe