When Lily Lurkers are immobile, they produce a huge amount of lag. Having a few "camouflaged" lurkers loaded at once often severely lowers global ticks per second. I believe a similar problem was found in Mowzie's Mobs "Ferrous Wroughtnaught" (which also becomes completely immobile when it has no current target), but the developers found a solution and optimized its code.

Perhaps you could look into something similar?