I've been doing some experiments on my old hardware to determine the overhead 4096 bit keys need. Generating a 4096 bit key takes around 20 minutes. Encrypting a ~20k message with a 4096 bit key takes less than 2 seconds, using a keyring with only one key. Encrypting with same setup using my default keyring (2000+ keys) takes around 15 seconds. Conclusion: Long key sizes are not an important gpg performance issue.