They didn't increase the difficulty so much as simply fixing a bug where AI were simply too easy. In the training demos before release and up to now, leaving a frame limiter off would cause the game to devote more resources to generating frames instead of handling your enemy's AI. They fixed this. So essentially, people who used features like VSync won't be affected as much as others who left VSync off most of the time. Those who had VSync turned off were simply playing the game on easy mode before the recent fix.