a) Nuke 7 is not as slow
b) The Silhouette Nuke exporter has had many new features added to it over the last several months to increase speed and flexibility.
Specifically, a new exporter feature places the shapes from each layer into its own Nuke node and connects these nodes together in a flow. Nuke is sped up because editing one node contains only the shapes from that Silhouette layer.
The new exporter is backwards compatible with later version 4 Silhouette.
Silhouette V5 was released yesterday (btw).
Disclaimer: I am part of Silhouette.