As we know, Composer isn't the fastest tool in the world. But it does its job well.
Unfortunately, with custom repositories, Composer is very slow because it reads each repository composer.json file (run a composer update -v, you'll see).
The only way to increase the Composer speed, is to install a proxy to host our custom repositories : Satis can do this, and it's open-source.
For more information about the Satis installation/configuration, please read the official documentation.
With 10 customs repositories, Composer took about 2/3 minutes to proceed the update command and only ~10 seconds after than Satis was installed.