Oh boy, there is a significant amount of information out there regarding lag and its causes. As far as I know, main causes of lag are size of world, size of embark, pathing calculations, number of items on the map, heat calculations and liquid flow calculations. I'm pretty sure there is more missing. Make sure to read up if you are at all concerned over poor FPS. But that said, the latest versions of DF are noticeably laggier than the ones preceding it
(34.11 is more lag friendly than 40.24 onwards due to lack of active world gen and multi tile trees for instance.)What is relevant in your case is pathing calculations. Your friend is right in the sense that 3 tile hallways are significantly better for pathing instead of dwarves trying to climb over each other in a narrow 1 tile corridor. It's like the difference between a 3 lane road versus a 1 lane road. You are far more likely to get a congestion on a 1 lane road. However, I'm no expert on whether this is a
significant source of lag or not. What is the FPS like in your fortress?
Personally I use 1 tile doors and 3 tile wide hallways along with 3x3 stairs for efficiency but I do like my aesthetics
Well, different people different strokes, what looks nice to one person, may not look that nice to another person
To be honest, I don't see why you both can't mesh your sense of aesthetics with his need for efficiency. But hey, play the game the way you want it to