[Plasma-dgm] 15% slower indexing in DGM after MB-32433 change

Test:
Reproduced only with indexing test we have as part of Aggregations test set.
Its 10 Indexes, Initial indexing, DGM, TPS-DC dataset
http://showfast.sc.couchbase.com/#/timeline/Linux/n1ql/aggregation/Indexing

The code change causing this behavior:
https://github.com/couchbase/plasma/commit/53c7714a7ff072a76f202bc79be77b363c78aec8

Comparison of 6.0.1-2019 and 6.0.1-2020:
http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=arke_601-2019_create_indexes_d2c0&snapshot=arke_601-2020_create_indexes_e6d0

For some reason, only one of the indexes gets affected (looks like):

Logs and test:
http://perf.jenkins.couchbase.com/job/arke-dgm/59/console