Could you just do the build in /dev/shm?
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Though from what I gather form the story, part of the spedup comes from how android composes their build stages.
I.e. speeding up by not downloading everything only helps if you don't need everything you download. And adds up when you download multiple times.
I'm not sure they can actually provide a speedup in a tight developer cycle with a local git checkout and a good build system.