If package is enough huge and have compressed files inside, decompressing them will improve game experience, but if package havent so compressed files ( just like a "tar" file), then uncompressing them will not succeed, even being a very huge file. It is a developer strategy for saving space, or to cover most market computers. Files are compressed into another files (pk4, wad, pak, whatever) only to avoid PC to slowdown searching sources when playing game. Making them to be together in same memory area (ram or swap) will make game to execute faster. Thats a very important development when creating a videogame. If that made D3 to be "slow" in a lot of systems, they have solved this already in newer expansion. U will need to find another workarounds to execute faster...sorry.
Just a how to.
Good luck!