Save many metafields with a single request

There does not seem to be a way to attach metafields locally, and then simply save the product with the metafields along with it. I'm forced to use add_metafields() or Metafield().save(), both of which adds a single metafield per API call. This is very slow.

If I take a product with 50 metafields, and save it as a new product, it saves almost instantly, like so:

p = shopify.Product.find()[0]
print len(p.metafields())
> 50
p.id = None
p.images = None
p.save()

How can I attach metafields to a product in the same way they are attached when they are fetched?

Start by tracing Product.save(), add_metafields(), and Metafield().save() to understand how fetched metafields are represented and persisted. The work is done when locally attached metafields can be saved with a new product in one API request, with coverage for the reported multi-metafield case.