In a nomad game I played as the Macedonians, I started building my CC almost as soon as I spawned—just after walking a short distance to an apple tree.
My Persian opponent didn’t start building their Town Center until about 30 seconds after I did, yet they finished earlier than I did. I checked the civilization overview and saw that Persians get a 20% reduction in build time for Barracks and Stables—but not for the CC. He had athenians team bonus, but again athenians bonus should give only tech upgrades bonuses, not building according to civ overview.
Additionally, during local testing, I noticed that the construction bonus is implemented by speeding up the countdown timer—meaning that when construction starts, the initial build time in seconds is the same as for other civilizations, but the timer ticks faster.
This is misleading. It would be clearer if the game instead recalculated the build time in seconds at the moment construction begins (applying the civilization bonus upfront) and then applied any additional bonuses.