Compare https://evergreen.mongodb.com/task/mongodb_mongo_master_enterprise_rhel_62_64_bit_coverage_jsCore_c065df4a1dae7fe0b73d7a85d13ff2beb47b459d_19_01_31_00_56_27 with the following commit that turned on v3, https://evergreen.mongodb.com/task/mongodb_mongo_master_enterprise_rhel_62_64_bit_coverage_jsCore_a285618a35742923e9e21fa2df4434406b121a4e_19_01_31_02_10_29

geo_array1.js took 42 seconds prior to the commit and 121 seconds after the commit. Most illustrative is the log line for the geo index build:
(prior to v3)
[MongoDFixture:job0] 2019-02-22T19:50:15.226+0000 I INDEX [conn55] index build: inserted 30000 keys from external sorter into index in 2 seconds

(after v3 upgrade)
[MongoDFixture:job4] 2019-02-22T19:55:54.009+0000 I INDEX [conn4] index build: inserted 30000 keys from external sorter into index in 11 seconds