[Question] What technologies could cause world GDP doubling times to be <8 years?

Some people think GDP is a good metric for AI timelines and takeoff speeds, and that the world economy will double in 4 years before the start of the first 1-year doubling period, and that AGI will happen after the economy is already growing much faster than it is today.

Other than AGI, what technologies could significantly accelerate world GDP growth? (Say, to a doubling period of <8 years, meaning the whole world economy grows 9%+ per year, significantly faster than the fastest-growing countries today.)

I find myself struggling to think of plausible answers to this question. Here are some ideas:

--Cheap energy, e.g. from solar panels or fusion

--Cheap resources, e.g. from asteroid mining, undersea mining, automated mines...

--Robots and self-driving cars make transportation and manufacturing cheaper

--3D printing? Idk.

--Narrow AI? Seems like the most plausible answer, but narrow AI doing what, exactly? Driving cars? Manufacturing things? Already discussed that. Inventing new products? OK, but in that case won’t they also invent AGI?

My problem is that while all of these things seem like they could be a big deal by ordinary standards, they don’t seem like that big a deal. Looking back over US economic history, it seems to my quick glance that growth rates haven’t changed much in 200 years. (!!!) But over that time energy, resources, etc. have gotten lots cheaper in the USA, and all sorts of new tech has been developed. Worldwide, it looks like the last time annual GWP growth was less than half of what it is now (excluding the Great Depression) was… 1875! (At least according to my data, would love to see a more thorough investigation of this). The world looked hella different in 1875 than it does now in 2020; doubling world GDP growth rates again seems like a pretty tall order. I believe that AGI could do it, but what else could?