I dug a bit in the code of create() in orm.py, and found an interesting value to pass in context:

AccountAccount.create(values, context={'defer_parent_store_computation': True})

This seems to defer the computation of parent_left and parent_right until later (and each account creation now only takes ~40ms. Unfortunately, I can't seem to be able to call _parent_store_compute via erppeek because it is not a public method...

So a solution is to use erppeek.get_pool(dbname) to get a Registry instance, tell that instance that the model needs reparenting and call do_parent_store:

registry = erppeek.get_pool('db_name')
registry._init_parent['account.account'] = True
cr = registry.db.cursor()
registry.do_parent_store(cr)
cr.commit()
cr.close()