Mongodb - Best Practice for TTL index on huge dataset
The rolling index build procedure is recommended. Here is what they say in the doc:


  To minimize the impact of building an index on replica sets and sharded clusters, use a rolling index build procedure as described on Build Indexes on Replica Sets.


for more info:

https://docs.mongodb.com/manual/tutorial/build-indexes-on-replica-sets/
https://docs.mongodb.com/manual/tutorial/build-indexes-on-sharded-clusters/
https://docs.mongodb.com/manual/core/index-ttl/
https://docs.mongodb.com/manual/reference/method/db.collection.createIndex/#db.collection.createIndex