That presumption seems to be precipitated on the theory that a computer intelligence won't "grow" or "learn" any faster than a human. Once the essential algorithms are developed and the AI is turned loose to teach itself from internet resources, I expect it's actual growth rate will be near exponential until it's absorbed everything it can from our current body of knowledge and has to start theorizing and inferring new facts from what it's learned.
Not that I expect such a level of AI anytime in the near future. But when it does happen, I'm pretty sure it's going to grow at a rate that goes far beyond anything a mere human could do. For one thing, such a system would be highly parallel and likely to "read" multiple streams of web data at the same time, where a human can only consume one thread of information at a time (and not all that well, to boot.) Where we might bookmark a link to read later, an AI would be able to spin another thread to read that link immediately, provided it has the compute capacity available.
The key, I think, is going to be in the development of the parallel processing languages that will evolve to serve our need to program systems that have ever more cores available. Our current single-threaded paradigms and manual threading approaches are far too limiting for the systems of the future.