Please consider adding the user-defined setting SWIFT_WHOLE_MODULE_OPTIMIZATION=YES flag in XCode, this will speed up compilation time drastically. Don’t let the optimisation level fool you, because that one should stay on “None”, and not be changed to “-whole-module-optimization”.
See this Hacker News thread https://news.ycombinator.com/item?id=13214431, with Uber (https://www.skilled.io/u/swiftsummit/swift-with-a-hundred-engineers) and Zalando (https://tech.zalando.com/blog/improving-swift-compilation-times-from-12-to-2-minutes/) mentioning it too.