How do I stop Xcode 9 from continuously building or at least make it not run build scripts?

Open any .storyboard file in your project and go to the Editor menu, uncheck 'Automatically Refresh Views'.

After upgrading the Xcode version to Xcode 9.0, I also faced a similar issue and my CPU usage was jumping upto 50% just because of Xcode. However I could solve it by following the steps below.

In the Toolbar go to: File -> Project Settings / Workspace settings change Build System to Standard Build Settings under Shared Workspace Settings. Also under Per-User workspace Setting change Build System to Standard Build System.

The release notes also say the following.

"Xcode 9 includes a new build system written from scratch in Swift. It is designed for higher reliability, and it catches project configuration problems that the standard build system does not. The performance of the build system (not including compilers, linkers, and other build tools) has been dramatically improved."

So there could be a possibility that there are still bugs in the new build system.