Node: v5.4.1
OS: OS X 10.11.5 / Darwin Stevens-MBP 15.5.0 Darwin Kernel Version 15.5.0: Tue Apr 19 18:36:36 PDT 2016; root:xnu-3248.50.21~8/RELEASE_X86_64 x86_64

I've narrowed it down to the detect-port library used. I tested out the library's CLI and got similar results.

Added a timer to the start script:

console.time('detect') detect(DEFAULT_PORT).then(port => { console.timeEnd('detect') ... })

Result of running npm run start

/Users/steven/my_projects/autocomplete steven@Stevens-MBP% npm run start > autocomplete@0.0.1 start /Users/steven/my_projects/autocomplete > react-scripts start detect: 75279.878ms