higher latency / lower throughput of GROUP_BY query with partitioned index

Index : 
CREATE INDEX by_capped_small ON `bucket-1`(capped_small, year, coins) PARTITION BY HASH(year);

Query (Showfast -> N1QL -> Q5-7 -> CI6):
SELECT year, SUM(coins) AS coins FROM `bucket-1` WHERE capped_small > $1 AND capped_small < $2 GROUP BY year

Parameters:

Request plus, partitioned index
Looks like only this combination is affected. But its hard to tell for sure because we don't have many tests on partitioned index.

Behavior:
Latency has increased and Throughput went 30% down since build 6.5.0-1489.
Comparison between 6.5.0-1488 and 6.5.0-1489:

http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=iris_650-1489_access_1cce&snapshot=iris_650-1488_access_1160

In addition to higher latency this graph might be interesting:

It's hard for me to track the particular change causing this because the 1489 was a massive merge:

 goproj/src/github.com/couchbase/indexing changed from c6e909a7802359cbc98ed800b7bcb9920d4941a1 to master
[+] 06062ae Merge remote-tracking branch 'couchbase/unstable' into HEAD
[+] 97dd385 MB-31600: cherry-picking query change on top of alice
[+] 98942af MB-31600 Handling secondary indexing on binary docs
[+] 1a06759 MB-31701: Enable parallel aggregate scan by default
[+] f330e65 MB-31673 nitro: Add definition of debugMarkFree to compile on non-amd64
[+] 4a005c0 MB-30207: Fix SUM aggregate query for large integers
[+] 9aecec9 control and data channel separation in projector
[+] 492ae03 MB-31577: Use httpMux with indexer https server
[+] 5a37a8c MB-31502 use waitPersist in RollbackToZero 

goproj/src/github.com/couchbase/plasma changed from 380d862cbf7b8975b3b1cb7ec69b3264beca591c to master
[+] 0b606e3 Merge remote-tracking branch 'couchbase-priv/unstable' into HEAD
[+] 5e0a14f MB-30731: Add lss_recoverypt_size, lss_maxsn_size stats

My bet would be on this one: https://github.com/couchbase/indexing/commit/1a06759c24975940f5e277fbcb106f114e49acd9

Logs:

Index:
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-iris-12635/172.23.100.45.zip

Query:
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-iris-12635/172.23.100.55.zip

Data:
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-iris-12635/172.23.100.70.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-iris-12635/172.23.100.71.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-iris-12635/172.23.100.72.zip
https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-iris-12635/172.23.100.73.zip