I agree performance can mean different things. However, performance means “speed” in the post and in the following sentence in the documentation for shared_memory module: “Sharing data directly via memory can provide significant performance benefits compared to sharing data via disk or socket or other communications requiring the serialization/deserialization and copying of data.”

As for the cause for the slowdown, the reason you state is possible but, from examining the code, I think the reason identified Jake Northey is the most likely cause.