defaults read com.apple.dt.Xcode > ~/Desktop/XcodeDefaults.plist

If you delete/move the current plist, Xcode will write a fresh one next time you run it.

killall Xcode
mv ~/Library/Preferences/com.apple.dt.Xcode.plist ~/Desktop/XcodeDefaults.plist
open -b com.apple.dt.Xcode

With Xcode 13.3 the build system and Swift compiler have a new mode that better utilizes available cores, resulting in faster builds for Swift projects. The mode is opt-in, and you can enable globally with the following user default:

defaults write com.apple.dt.XCBuild EnableSwiftBuildSystemIntegration 1

Source @BenchR

defaults write com.apple.dt.Xcode ShowBuildOperationDuration -bool YES

defaults write com.apple.dt.Xcode BuildSystemScheduleInherentlyParallelCommandsExclusively -bool YES

Xcode 9.3 now runs more Swift build tasks in parallel with other commands. This may improve build times for Swift projects, but may also increase memory use during the build. This feature can be disabled from Terminal by setting a user default with

defaults write com.apple.dt.Xcode BuildSystemScheduleInherentlyParallelCommandsSerially -bool YES

defaults write com.apple.dt.Xcode PegasusMultipleCursorsEnabled -bool YES

defaults write com.apple.dt.Xcode IDEBuildOperationMaxNumberOfConcurrentCompileTasks `sysctl -n hw.ncpu`

Where sysctl -n hw.ncpu gives you the number of CPU threads.

defaults write com.apple.dt.Xcode PBXNumberOfParallelBuildSubtasks `sysctl -n hw.ncpu`

Where sysctl -n hw.ncpu gives you the number of CPU threads.

Keep in mind that disabling indexing will disable refactor code action so you probably won't be able to generate protocol conformances or memberwise initializers

defaults delete com.apple.dt.Xcode IDEIndexDisable
defaults write com.apple.dt.Xcode IDEIndexEnable -bool YES

defaults delete com.apple.dt.Xcode IDEIndexEnable
defaults write com.apple.dt.Xcode IDEIndexDisable -bool YES

defaults write com.apple.dt.Xcode IDEIndexerActivityShowNumericProgress -bool YES

This will show you why a particular file is having trouble being compiled for indexing.

defaults write com.apple.dt.Xcode IDEIndexShowLog -bool YES

defaults write com.apple.dt.Xcode IDEShowPrebuildLogs -bool YES

If set to SourceKit will write a log to /tmp with all the details of what it is doing while indexing. A lot of people find they have header hygiene problems or module problems that happen to work while building in certain configurations but aren't actually correct, resulting in missing modules or broken headers from the indexer's point of view. May not work anymore.

defaults write com.apple.dt.Xcode IDESourceKitServiceLogLevel -int 3 

Also you can do this temporarily by pre-pending this environment variable when starting Xcode:

SOURCEKIT_LOGGING=3 /Applications/Xcode.app/Contents/MacOS/Xcode

Source AppCode under the hood - Aydar Mukhametzyanov - NSSpain 2021

Deactivates the Main Thread Checker. Found in Xcode 12 release notes

defaults write com.apple.dt.Xcode DVTDisableMainThreadChecker 1

defaults write com.apple.dt.Xcode ShowDVTDebugMenu -bool YES

Prevents Xcode from automatically restoring the last open project. This enables running multiple Xcode versions for different projects.

defaults write com.apple.dt.Xcode ApplePersistenceIgnoreState -bool YES

defaults write com.apple.dt.XCBuild EnableDebugActivityLogs -bool YES

This slows down the build system & litters DerivedData//Build/Intermediates.noindex), generally should only be enabled when trying to capture a trace for incremental build debugging purposes.

defaults write com.apple.dt.XCBuild EnableBuildDebugging -bool YES

Make Xcode's Assistant aware of your ViewModels, Views, etc. Found by @peterfriese

defaults write com.apple.dt.Xcode IDEAdditionalCounterpartSuffixes -array-add "ViewModel" "View" "Screen"

Do not move files when you restructure things in an Xcode project. Found by @steinpete

defaults write com.apple.dt.Xcode IDEDisableStructureEditingCoordinator -bool YES 

Stop Xcode from reopening files on launch. Found by @SmileyKeith

defaults write com.apple.dt.Xcode IDEDisableStateRestoration -bool YES

Homebrew's default prefix/install path can point to different locations depending on your system.
Xcode by default only looks for /opt/brew and /usr/local.
You can adjust that location by writing the following default.
To get your Homebrew prefix call brew --prefix. Source @NeoNacho

defaults write com.apple.dt.Xcode IDEHomebrewPrefixPath -string <BREW_PREFIX>

Specify the directory to which remote source packages are fetch or expected to be found. xcodebuild has the same option as clonedSourcePackagesDirPath. Source @bguidolim

defaults write com.apple.dt.Xcode IDEClonedSourcePackagesDirPathOverride -string <PATH>

defaults write com.apple.iphonesimulator AllowFullscreenMode -bool YES

Creates an XCBuildData folder in ~/Library/Developer/Xcode/DerivedData/<your target>/Build/Intermediates.noindex/ which contains debugging info for xcodebuild.

defaults write com.apple.dt.XCBuild -bool YES

... provided Xcode is set to default folder locations. This is a quick win and helps to get back gigabytes of storage space. Do this regularly.

rm -rdf ~/Library/Developer/Xcode/DerivedData/*
rm -rf $(defaults read com.apple.dt.Xcode.plist IDECustomDerivedDataLocation)

... provided Xcode is set to default folder locations.

CACHE=$(getconf DARWIN_USER_CACHE_DIR)
rm -rdf ${CACHE}com.apple.DeveloperTools
rm -rdf ${CACHE}org.llvm.clang.$(whoami)/ModuleCache
rm -rdf ${CACHE}org.llvm.clang/ModuleCache
rm -rdf ~/Library/Caches/com.apple.dt.*/*

TMP=$(getconf DARWIN_USER_TEMP_DIR)
rm -rdf ${TMP}*.swift
rm -rdf ${TMP}ibtool*
rm -rdf ${TMP}*IBTOOLD*
rm -rdf ${TMP}supplementaryOutputs-*
rm -rdf ${TMP}xcrun_db
rm -rdf ${TMP}sources-*
rm -rdf ${TMP}com.apple.dt.*
rm -rdf ${TMP}com.apple.test.*

• DevCleaner If you want to reclaim tens of gigabytes of your storage used for various Xcode caches - this tool is for you!
• Kintsugi A tool to automatically resolve Git conflicts that occur in Xcode project files
• BuildTimeAnalyzer A build time analyzer for Xcode
• XCLogParser A tool to parse Xcode and xcodebuild logs stored in the xcactivitylog format
• XCMetrics A tool to collect Xcode build metrics and improve developer productivity