At monarch the AI will build and research 10% faster, yes. That is not significant. But when they found their first city, where you will have your capital and worker, they will immediately spawn FREE settlers, workers, and military units (not much in monarch, but rapidly increasing with difficulty level). This means you are crawling up from being underdog while building 10% slower than they do. Their 2 cities pumping out shields at 90% your time is much stronger than your 1 city in the Ancient era.