GnuPG appears to have quite slow symmetric decryption speed compared to encryption and other tools. This then shows up when users benchmark GnuPG against other tools (for example here: https://www.osso.nl/blog/encryption-decryption-speed-gnupg-openssl/).
Problem appears to be the extra hash contexts enable at: https://git.gnupg.org/cgi-bin/gitweb.cgi?p=gnupg.git;a=blob;f=g10/mainproc.c;h=1ee5b9a6e4c1e49ffb4d49024eddd79d1b96a229;hb=HEAD#l917.
With those manually disabled, symmetric AES256 decryption speeds up by 3x on current generation x86-64 cpu. I think it would be good if we could find way to disable those hash contexts when processing data encrypted with current GnuPG versions. Currently we needlessly waste users' time and computation power to support handling pre-2440 input. Also performance advances in libgcrypt go pretty much unnoticed because this extra computation. Maybe we could use some heuristic (packet version? etc) to avoid enabling extra SHA1 & RMD160 hashing?