Couchbase server is taking too much time to execute simple n1ql query

I am using Delete query e.g.

delete from buckethelp a where (a.type = ‘point’) and DATE_FORMAT_STR(a.time, ‘1111-11-11’) = ‘2017-11-15’

The records deleted around 60 K.

If I run delete query multiple times (e.g. delete data for 4 to 5 days with above query), at the same time Sync_gateway adding documents in the bucket. Does it cause any problem with Couchbase, and because of that the cocuhbase stops executing select queries ?

Couchbase version I am using is 4.6.0