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Their shader is doing software 64 bit float emulation, including trignometric functions.

That city graphic is really precise.

(edit: it looks like the 64 bit math is used only for the map projection part of the shader... so it's not quite as ridiculous as it might seem, my haswell system is limited by CPU, not by vertex shader performance)

What should I google to find out more on this topic? Why do they need double precision if they aren't doing computation? Is it actually visible to the human eye?

They are feeding in lat/long coordinates to the shader, and then doing the map projection there. Effectively they want lots of precision, but also the ability to represent coordinates anywhere in the world, in their shader. The alternative is to precompute the projection before putting it into the shader.

Getting around 30fps on a 2015 Macbook Air. Might be due to yours rendering at retina resolution.

I get 20 fps on Safari, 30 fps on Chrome on my Mid-2013 MacBook Air, still running El Capitan.

Just for reference, I'm getting ~10 fps on my Nexus 9 with Chrome.

and its all CPU bound limited by single threaded js. GPU is sleeping <100MHz with barely any usage.