Slowness from reindexing the database when opening database

From https://issues.couchbase.com/browse/CBL-2155, it seems like there are some changes to fl_value that requires to reindex the indices in kv_default. This has a potential to slow the opening database operation first time. This can cause all existing apps to be halt for a moment. I'm creating this ticket to see if there is a way to mitigate this issue or not. 

1. Can we measure the impact of this before taking action on the actual devices (e.g. iPhone and Android)?

2. Is there a way to make fl_value falling back to do the old value?

3. Is is possible to reindex in the background instead of hanging?