The zombie lag from failing path searches has NOT been fixed yet in 1.7.2 (though the changelog tells us to believe so), instead a sloppy workaround was done, that doesnt even cover all cases. We still get horrible lag when 1) jumping alot while been chased by zombies, 2) standing in an unreachable position (ie. 1x1 tower) or 3) flying in gamemode 0 above zombies (fly ability granted by a plugin). Applying a variant of the workaround that we had for 1.6