I was a build engineer in a previous life. Not for Android apps, but some of the low-effort, high-value tricks I used involved:
* Do your building in a tmpfs if you have the spare RAM and your build (or parts of it) can fit there.
* Don't copy around large files if you can use symlinks, hardlinks, or reflinks instead.
* If you don't care about crash resiliency during the build phase (and you normally should not, each build should be done in a brand-new pristine reproducible environment that can be thrown away), save useless I/O via libeatmydata and similar tools.
* Cross-compilers are much faster than emulation for a native compiler, but there is a greater chance of missing some crucial piece of configuration and silently ending up with a broken artifact. Choose wisely.
The high-value high-effort parts are ruthlessly optimizing your build system and caching intermediate build artifacts that rarely change.