Packet.net offer an ARMv8 server with 96 cores for $0.50/hour. I signed up and tried building Libreoffice to see what would happen. Debian isn't officially supported there yet, but they offer Ubuntu, which suffices for testing the hardware.
Final build time: around 12 hours, compared to 2hr 55m on the official arm64 buildd.
Most of the Libreoffice build appeared to consist of "touch /some/file" repeated endlessly - I have a suspicion that the I/O performance might be low on this server (although I have no further evidence to offer for this). I think the next thing to try is building on a tmpfs, because the server has 128GB RAM available, and it's a shame not to use it.