I buy that maintaining a CI setup is not a trivial thing, but if speed is the only other concern, why not just...spin up faster CI runners? De-centralizing tes

I buy that maintaining a CI setup is not a trivial thing, but if speed is the only other concern, why not just...spin up faster CI runners? De-centralizing test running also makes it difficult/impossible to e.g. collect test analytics (like Buildkite can do natively) to help gain insight into flaky tests, how code changes impact test duration, etc., and visualize those things over time, especially if developers are using non-homogenous hardware. That, and if your tests are not being run in CI, then what enforces that tests run and pass before a deploy occurs? Losing that guarantee is not great IMO.