Composer 2.0 was recently released with much improved performance in fetching and updating dependencies. If you are running projects on Drupal 8 or 9, you are going to want to switch as soon as you can.
On a recent project, we had taken a Pattern Lab based prototype and created a Drupal 8 theme from it. In order to track updates and stay independent of the work happening by the front end team, I added the Pattern Lab repo to the project as a git submodule.
Adding Composer packages to Drupal 8 is pretty easy.
There is a module called Composer Manager that can be used to add packages that modules require, but there are instances where you just need a package (and no need to create a module).