Moved out from T3598 :

(1.) issuing "gpg -o <Decrypted_file.tar.gz> -d <Encrypted_file.tar.gz.gpg>" in a Win 7 terminal is impracticably slow; decrypting a file with 35gb takes about 12h (or more)

I noticed similar behavior. The write of gnupg can probably be optimized on Windows by using larger chunk sizes or by not waiting until a write is synced or so.
In Kleopatra we could write the decrypted data into a buffered device and get a performance improvement this way.