You're all forgetting here about the most important factor for turn times: difficulty level. I haven't tested Civ6 extensively since I simply don't really like the game but it's the case in here as well as it was with Civ5. In Civ5, turn times were fast to moderate on difficulty settings up to emperor. With immortal and above, turn times after industrial era were dreadful. From what I've seen in Civ6, as long as you don't go above emperor, you can play on huge maps just fine, but above emperor, when there are tons of units in game, you'll be suffering past mid-game even on standard-sized maps.