Monorepos enhance developer velocity but can lead to conflicts and deployment issues when multiple developers work within the same repository. A merge queue, which merges changes in a first-in-first-out order, helps create predictable releases and reliable software. However, long CI times can delay the queue, especially as the volume of pull requests increases. To address this, Parallel CI is introduced to the Graphite merge queue, allowing CI checks to run in parallel for multiple stacks without sacrificing correctness. This improvement significantly reduces merge times, with some early adopters experiencing up to 2.5 times faster merges. Parallel CI maintains correctness by ensuring each merged pull request passes CI on its own, while also offering the option of batch merging for higher throughput at a lower CI cost. The system allows teams to configure parallelism settings to optimize their merge process, balancing between correctness and efficiency.