It's almost like clockwork, whenever the manta gets within distance the game stutters and lags for a full second or two and performance nosedives. I want to believe that this is partially due to LOD loading and possibly the number audio channels getting spammed when it comes near.
The other thought is that it's annoyingly painful to watch it fly through buildings. It would be nice if there was some collision avoidance AI helping it to avoid players structures. The higher you build, the higher it flies to avoid those structures.
Another solution: Let me kill it and be rid of it for good.