It's also super jarring because it's like the AI suddenly wakes up at a set point. This is most noticeable if you play an early warmonger. You will steamroll your neighbors up until fourth era when the AI starts producing walls of units. If your looking to play optimal this means pounding the AI right at the start of third era to force them into vassals before they kick into overdrive. Your vassal AI slaves get the same bonuses as the free AI civs. (and unless your completely inept will never rebel against you) Not saying this is fully good or bad. It does feel a tad artificial though.