What you're saying, I think, is that the title and question fail to recognize the difference between the basic required step of "decompressing something" and the extended sequence of "extracting files from an archive and writing them to disk".

You simply can't read plaintext without doing the work of decrypting and/or unzipping the original file, because you don't HAVE the plaintext until you do. It sounds to me like "how can I avoid the use of temp files and the slowness of the HDD?" is the real question here.