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 have seen the glitch actually, in 1.4.6, I haven't noticed it in the newest snapshot yet though.
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.
Creepers, skeletons, sheep etc. except for spiders are all affected by this glitch.
The leaves break stairs too.
I've had the same problem testing rails. It only seems to glitch when the player rides the minecart. If he doesn't ride, the glitch doesn't start. Version 1.4.4.
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.