What do you mean when you say "test runs are about 20-25% faster on ARM vs x86"? Are you comparing similarly priced systems, specific processors, or what?
CircleCI offers machine executors, which I assume are basically just EC2 instances (in the past they were hosted on GCP), and for example using 'large' executor has the same per second billing, but large ARM instance is ~25% faster than large x86 instance.