First of all try running your command with the -o -vvv flags. This would show each step in the composer process and you will notice where it slows down.
composer update -o -vvv
If it's showing slowness for http://repo.packagist.org you can try forcing the switch to https
composer config --global repos.packagist composer https://repo.packagist.org
I had to do this inside my docker container.
If that does not work try a specific mirror (UK mirror in this case)
composer config repositories.packagist.org composer https://repo-eu-uk-1.packagist.org
More mirrors can be found on the packagist website or Webysther/packagist-mirror
To reset the specific mirror you can run the following afterwards
composer config repositories.packagist.org --unset