Slow updating of composer dependencies, despite --prefer-dist flag

This problem is often related to xdebug being loaded in your CLI environment. (It doesn't matter if xdebug is enabled or not.)

You can check whether xdebug is enabled using one of the followinc commands.

// Unixphp -m | grep xdebug// Windowsphp -m | findstr xdebug

Further information on what operations take so long can be gained by enabling maximum verbosity and profiling information. (Replace install with update if you are updating the packages.)

composer install --prefer-dist -vvv --profile

Factors that can slow down Composer:

• As pointed out, xdebug can affect the performance of Composer. Running composer diagnose will also warn you about this.

• Running update instead of install. People too often just run update constantly. This makes Composer go through the entire dependency resolving process, regardless of whether or not anything has changed. When you run install, Composer takes the requirements directly from your .lock file, skipping the dependency resolving process. You should only run update during the development lifecycle of your application. And even then, it's not something you have to run daily usually.

• If you have a specific dependency that you update frequently yourself, you could try simplifying the process by running composer update vendor/package --with-dependencies instead.

• Setting minimum-stability to dev. This greatly expands the amount of possibilities that the dependency resolver has to consider. You should almost never lower the minimum-stability to dev unless you absolutely have no other choice. Look into alternatives, such as temporarily using a inline @dev flag.

Seems like the problem was resolved, but this might help someone.

Whenever I ran composer install or update, it took more than 10 seconds just to fetch the https://packagist.org/packages.json file. Eventually I found out that the problem was related to IPv6, since fetching files from IPv4 sites took less than a second.
The problem is that my ISP doesn't support IPv6, but I had it enabled in my ethernet properties. After I unticked Internet Protocol Version 6 (TCP/IPv6) in my network settings, install/update speeds drastically improved (dropped from 200+ seconds to like 10)