これ気になる https://twitter.com/usagimaruma/status/938062738550427653 リリースノートが間違ってて、正しい試し方は defaults write com.apple.dt.Xcode BuildSystemScheduleInherentlyParallelCommandsExclusively -bool NO らしいよ。 https://twitter.com/rballard/status/938216933303885824 (edited)

The Xcode 9.2 release notes mentioned an experimental feature that may improve Swift build times, enabled with the "BuildSystemScheduleInherentlyParallelCommandsExclusively" user default. Unfortunately, we documented this incorrectly. Set it to "NO", not "YES", to try this out.