Today we're vertically scaling builders, so multiple builds run on the same large EC2 instance. The instances support processing multiple Docker builds at once, and thanks to BuildKit can even deduplicate work between multiple parallel builds, so like if two simultaneous builds share some steps, it can handle coordinating so those steps run just once.

We have plans to expand to more horizontal scaling with tiered caching, so we can keep the speedups we see today but further increase potential parallelism.