N1ql SUM aggregate taking little time

My query in which i am taking the sum of customers total paid . i have around 100k invoices for the given customer right now.
My query is taking around 5 to 7 seconds to sum the total payments of given customer . is there any ways to
make it much faster.

MY QUERY

SELECT SUM(total_bill) AS total_bill, SUM(total_paid) as total_paid from stitchit_data_bucket where cus_id = ‘024420000000’ AND type = ‘invoice’

MY INDEX
CREATE INDEX def_cus_id_type ON stitchit_data_bucket(cus_id,type)

  • index - def_cus_id_type
  • index_id - 817d4e1db8bf21a4
  • index_projection
    • primary_key - true
  • keyspace - stitchit_data_bucket
  • namespace - default
  • spans
    • exact - true
    • range
      • high - “024420000000”
      • inclusion - 3
      • low - “024420000000”
      • high - “invoice”
      • inclusion - 3
      • low - “invoice”
  • using - gsi

CREATE INDEX dix1 ON stitchit_data_bucket ( cus_id , total_bill, total_paid) WHEE type = “invoice”

Please checkout https://blog.couchbase.com/create-right-index-get-right-performance/.
If need help use Index Advisor https://index-advisor.couchbase.com