If you are seeing larger index creation times with increase in number of indexes, it is most likely due to index planner component (Couchbase Server - Index Planner for Global Secondary Indexes). For faster index creation times, you can disable planner using the below setting:
curl -u : http://<indexer_ip>:9102/settings -X POST -d ‘{“queryport.client.usePlanner”:false}’
Please note that disabling planner can lead to sub-optimal distribution of index load on the cluster.
Thanks,
Varun