time in front of any shell command and once it finishes, it will show how long it took to run.
This could be useful when making tweaks to your build system and want to know what impact it has on the running time.
time npm run build will gives me the following output:
real 1m15.659s user 1m26.313s sys 0m7.911s
See this Stack Overflow answer for more details about the difference between real, user and sys