I added screenshots with the newest snapshot showing the fps with and without the walls. The client is not modded, but the problem still persists when there are a lot of cobblestone walls stacked together, getting worse with a bigger stacking.
Yes I had mcpatcher installed and didn't realize this was a mod, sorry about that. However the problem still persists without any mods, the new (green)pictures use snapshot 13w04a, without mods.
I've only been on a single player instance and even the normal texture pack and client causes lag with fences and walls in the pattern.
It does, but it takes a riduculus amount of cobblestone walls to get me down to half fps on a superflat world. I added pictures. More like, I added cobblestone walls until I started "double-placing" them.