When a larger number of fences (normal or netherbrick) is present in a close proximity to the player interacting with blocks (placing, breaking, enchanting, renaming items,...) is almost impossible due to immense lag spikes (spikes are dependant on the number of fences of course) that occur.

Dev crash log:
http://pastebin.com/gf8P60Z8

I have also included a test world to demonstrate the bug.