Many tools will measure how long it takes for a computer to do something. Stopwatch.sh is one I keep up my sleeve for when none of those other tools fit.
I've developed a simple workflow to run builds, tests, and other operations during development that allows me to easily and efficiently examine results.
Ejecting your Create React App has major tradeoffs. Yes, it gives you more choices, but future configurations are time-consuming, expensive, and difficult.