Why older hardware benefits from different ciphers?

Sometimes I use my legacy hardware and try to do something useful with it. My latest "old" new acquisition with Intel Core 2 Duo runs FreeBSD uninterrupted for over 60 days now.

Recently I noticed, that file transfer speed drops to 192KB / s on a VPN connection. First I thought this might have been a internet issue. Very often I must use the internet on train or mobile link, but pieces still did not add up.

It turns out that the CPU was hogged at 80-100% usage trying to encrypt and decrypt data.

To my surprise I found out that the config was using a modern cipher variant AES-256-GCM which does not perform very well on older hardware. So I switched to CHACHA20-POLY1305, which works reasonably well.