I think it was his constant buying of silvers that slowed him down. He's diluting his deck, making it take longer to get back to his big money cards, and making his ventures less likely to chain.
Someone once posted on here that they thought you should minimize your silver buys on Colony games because they slow you down too much. I've followed that advice ever since, and it has seemed to work well for me. Silvers are very different in Colony games because even with a hand full of them, you can't buy your target, where as in a Province game, it only takes 4 to get what you're going for.