The Xcode 9.2 release notes mentioned an experimental feature that may improve Swift build times, enabled with the "BuildSystemScheduleInherentlyParallelCommandsExclusively" user default.
defaults write com.apple.dt.Xcode BuildSystemScheduleInherentlyParallelCommandsExclusively -bool NO
Note: According to the release notes it's an experimental feature which can "increase memory use during the build"
