The work in REP-2039 added the ability for users to pass a new parameter to the /start endpoint to skip building non-essential, user built indexes. This has uses for analytics and speeding up migrations. Some indexes will still be built.

The scope: https://docs.google.com/document/d/1_EWHqLl3IXri0gUJ1Sofvd6D8O_f12KwP8joqeFMQnA/edit# should be a good starting point on feature details.