I feel like this makes it sound worse than it is. I run El Capitan in a VM on a 6 year old ThinkPad with a 3rd-gen i5 and it's fine. I use Safari with developer tools to debug web stuff and I've not noticed any unpleasantness. I don't use iMessage so maybe it's particularly graphics-intensive or depends on a lot of GPU acceleration (would seem strange in a chat app). Video chat might be affected I suppose? Anyway it may well be worth trying so I didn't want people to be unduly put off.