Massive improvements in terms of both speed and memory usage with new Composer.

The difference depends on your use case, so while I've seen reports of improvements of over 50% to both in some projects, I cannot put an exact number on it.

As a side note to this, require/remove and partial updates are now much faster because Composer will now only load the metadata of the packages which are being changed.

Drush is a command line shell and Unix scripting interface for Drupal. Drush core ships with lots of useful commands for interacting with code like modules/themes/profiles. Similarly, it runs update.php, executes sql queries and DB migrations, and misc utilities like run cron or clear cache. Drush can be extended by 3rd party commandfiles.