Large namespace copy is slow

[johnny-morrice]

Namespace is our core datastructure and its a little slow to copy a big one.

This is more of an issue for our test suite than any real use-case I have found. In real life, we have many small namespaces that do not suffer from these issues. However, it would be best all round if the datatypes could be snappy, and it's not a hard problem to solve.