Haha seems like you ran into the exact same issues as I did! It works brilliantly with a concurrency of 2 on an early 2015 2-core i5 13" MBP with 16GB of RAM, and I'm even using the machine interactively while builds are processed. I'm using Sierra as a base but I've been building Mavericks, El Capitan and High Sierra ones too. VMs are 1 CPU / 4GB. I saw no gain in bumping up VMs to 2 CPUs.
https://gitlab.com/archmac/packages/pipelines