It doesn't do anything you couldn't be doing without it; so the answer might well be that it doesn't improve performance at all. It could vary from user to user.
It does encapsulate some mechanisms (which you might already be using) for making good use of autoloading and deferred evaluation of code, in order to avoid loading libraries any earlier than necessary; so users who weren't doing that might well see noticeable improvements to their start-up times after converting their configs to
My understanding of Emacs's package management is that Emacs automatically loads all installed packages.
Only the autoloads file for each package is
loaded during package initialisation. If you were to
load each individual elisp library provided by each package, that would take a lot longer.