Composer: El compositor es muy lento

Hoy en día, un comando composer install tarda mucho en finalizar. Ocurre tanto a nivel global como local.

El composer diagnose está todo bien.

Cuando ejecuto este comando (pero es lo mismo con un instalador local):

$ composer global require  hirak/prestissimo --profile -vvv

Obtengo el siguiente resultado:

[1.4MB/0.00s] Changed current directory to /home/vagrant/.config/composer
Failed to initialize global composer: Composer could not find the config file: /home/vagrant/.config/composer/composer.json
To initialize a project, please create a composer.json file as described in the https://getcomposer.org/ "Getting Started" section
Running 1.4.2 (2017-05-17 08:17:52) with PHP 7.1.5-1+deb.sury.org~xenial+2 on Linux / 4.4.0-31-generic
[1.7MB/0.01s] Reading ./composer.json
[1.7MB/0.01s] Loading config file ./composer.json
[2.0MB/0.01s] Checked CA file /etc/ssl/certs/ca-certificates.crt: valid
[1.9MB/0.02s] Executing command (/home/vagrant/.config/composer): git branch --no-color --no-abbrev -v
[1.9MB/0.03s] Executing command (/home/vagrant/.config/composer): git describe --exact-match --tags
[1.9MB/0.03s] Executing command (/home/vagrant/.config/composer): git log --pretty="%H" -n1 HEAD
[1.9MB/0.04s] Executing command (/home/vagrant/.config/composer): hg branch
[1.9MB/0.04s] Executing command (/home/vagrant/.config/composer): fossil branch list
[1.9MB/0.04s] Executing command (/home/vagrant/.config/composer): fossil tag list
[1.9MB/0.05s] Executing command (/home/vagrant/.config/composer): svn info --xml
[2.4MB/0.09s] Downloading https://packagist.org/packages.json
[2.4MB/7.84s] Writing /home/vagrant/.cache/composer/repo/https---packagist.org/packages.json into cache
[2.4MB/7.84s] Downloading http://packagist.org/p/provider-2013%24442e1d5fa0fdbf77855cc363b024994a3e92a2f5099e5138b9c5777ae486703c.json
[2.5MB/108.65s] Downloading http://packagist.org/p/provider-2013%24442e1d5fa0fdbf77855cc363b024994a3e92a2f5099e5138b9c5777ae486703c.json

Se bloquea en la última fila, pero es muy aleatorio, porque si lo dejo durante ~ 30 minutos, continúa buscando otros URI, pero se detiene aleatoriamente en otros URI y, a veces, tarda más de 30 minutos.

¿Alguna idea de por qué está pasando? Parece solo para mi computadora (tanto la máquina vagabunda como la local), pero si intento con otra máquina, funciona normalmente. ¿Qué puedo comprobar?