As per the curl man page, "Make curl display information on stdout after a completed transfer. The format is a string that may contain plain text mixed with any number of variables. The format can be specified as a literal "string", or you can have curl read the format from a file with "@filename" and to tell curl to read the format from stdin you write "@-"."
Run the following command fron inside the "curl" folder to get quick performance numbers for an API -
curl -w "@curl-format.txt" -s -k -o /dev/null <URL>
Example -
curl -w "@curl-format.txt" -s -k -o /dev/null "https://www.google.com"
Result -
response_code: 200
time_namelookup: 0.005137
time_connect: 0.009728
time_appconnect: 0.133735
time_pretransfer: 0.134052
time_redirect: 0.000000
time_starttransfer: 0.250884
-----------
time_total: 0.263603