N1QL queries very slow

Hello,

We use Couchbase Server Version: 4.1.1-5914 Enterprise Edition (build-5914)
Cluster with 4 nodes in production.
We use last version of Couchbase java client to make N1QL queries on this cluster : https://github.com/couchbase/couchbase-java-client

Part of the queries create timeouts on Couchbase side. They are visible in the administration panel, in analytics section (query_requests_5000ms). Only some of them are slow, when you run it multiple times, sometimes it’s very fast, sometimes it timeouts…

We use simple N1QL query, example : SELECT META(default).id AS couchbaseId, * FROM default WHERE type=‘alerte’ AND internauteId='74282378’
We have 18Gb RAM, only 136Mb in use, 225Mb of data… So very limited bunch of data…

I created primary index on the default bucket, and secondary index matching our queries, example : CREATE INDEX idx_alerte_internauteId_2 ON default(internauteId,type) USING GSI

If you can help me to figure out what’s going on…
Thank you so much !