Update - it finally took 3.5hrs to pull in the data. FPS is still super slow compared to its Enscape counterpart (as a benchmark) - both using dynamic lighting. In the end Datasmith generated 142,152 items for this import. City model I would expect to be as complex as a City Engine scene.
Is this to be expected or are there any optimisations I should be trying? Assume that light baking may also help reduce the overhead?