A LOT of man-hours went into minimising the impact of ads on load times, but you're tied into contracts with a lot of these advertisers, and if you don't keep up your end of the bargain by including their code where they want, and including their ads exactly where they want then they could and would cut what they paid you. It was a continual balancing act to keep cash flowing in, and trying to minimise the impact of those things on the user experience. At the end of the day we (the developers) were at the mercy of the contracts agreed between our company and the advertising companies, no matter what our opinion was of the negative performance, traffic and privacy implications.
Incidentally we found that during trials, providing subscription/ad-free models was just about as profitable as the advert stuffing route - but only for certain markets. People in the big cities were far more willing to pay a subscription for content, but the rest of the country expected their stuff for 'free'.