Xcode 9.3 (9E145)

Not sure which version you upgraded from, but speed has been an issue for a while now.

Xcode 9 is Unacceptable

Might be something you can try, tho - from the release notes:

https://developer.apple.com/library/content/releasenotes/DeveloperTools/RN-Xcode/Chapters/Introduction.html#//apple_ref/doc/uid/TP40001051-CH1-DontLinkElementID_1

Build System

New Features

• Xcode 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

  . (35551983)
• Added an option to optimizing by size (

  -Osize

  ) to the Optimization Level for the Swift compiler to the Build Settings pane of the Project editor. When this mode is selected the Swift compiler minimizes the size of compiled code. (36887476)The choice for compiling Swift code by file or by module moved from the Optimization Level setting to Compilation Mode, which is a new setting for the Swift compiler in the Build Settings pane of the Project editor. Previously this choice was combined with others in the Optimization Level setting. Compiling by file enables building only the files that changed, enabling faster builds. Compiling by module enables better optimization. (36887476)