Isn't that effectively a problem for any hourly based service? What's the incentive for a lawyer, accountant, developer, anyone to be as cost effective as possible? Surely the answer to that (happy customers, repeat business, referrals) would apply here as well. Even if hypothetically it wasn't as efficient as possible yet they still managed to deliver quality results, whats the problem? You're paying for results not effort, no?

User retention and product quality.

If we aren't efficient and cost-effective, we won't be able to retain users and our product experience will be severely harmed. Whether we are charging hourly or not, it must be the case that the value we offer the user is greater than or equal to the cost that the user is paying.

Needless to say, at Magic we hold ourselves to a very high standard when it comes to product quality and value. And, we use the product ourselves and we pay hourly.

Maybe in that scenario, "time spent waiting", they don't charge? Seems like they wouldn't want to waste those cycles, but I see your point with regards to incentivisation.