How to Use cURL to Measure Web Request Timings and Diagnose Performance Bottlenecks

cURL is a powerful web request tool that can also measure request timings.

Practice

Create a curl configuration file (~/.curlrc) with the following content:

# Content (long, can scroll)
-w "dnslookup: %{time_namelookup} | connect: %{time_connect}
appconnect: %{time_appconnect}
pretransfer: %{time_pretransfer}
starttransfer: %{time_starttransfer}
total: %{time_total} | size: %{size_download}"

Use the configuration with a curl command, for example: $ curl -so /dev/null https://www.zasag.mn The output will display timing values such as:

dnslookup:1.510
connect:1.757
appconnect:2.256
pretransfer:2.259
starttransfer:2.506
total:3.001
size:53107

Explanation

The following timing variables are reported:

time_namelookup – DNS lookup time.

time_connect – Time to establish a TCP connection to the web server.

time_appconnect – Time to complete the TLS handshake.

time_starttransfer – Time until the client receives the first byte of the response.

time_total – Total time until the client closes the connection.

These metrics help you understand the details of the request process and identify performance bottlenecks.

Content is adapted from Cloudflare’s blog post “A Question of Timing”.