Unindexed distinct queries are slower in 2.6 WRT 2.4, and this is noticeable both in single-threaded and multi-threaded tests I ran. An easy (single-threaded) way to reproduce it can be found attached.

27fb434eaebc35646cdadf7acac562aa47c06ac8 (HEAD as per today) output:

connecting to: test
Trial number: 0 1608
Trial number: 1 1603
Trial number: 2 1678
Trial number: 3 1604
Trial number: 4 1597

2.4.9 output:

connecting to: test
Trial number: 0 1273
Trial number: 1 1272
Trial number: 2 1269
Trial number: 3 1271
Trial number: 4 1266