Currently the assertion import job is always running on all Ubuntu Core 16 images. Due to the nature of the design it has to probe for disks/USB keys/etc which might be slow in replying (due to a slow controller, or because there is an additional (low speed) USB key with data for an application snap etc). This can slow down the boot process of a device.
There are many scenarios where consumer images will not use the assertion import feature at all (pre-configured at factory, pre-configured via cloud-init) or even consider it a security risk to allow access even through something signed …
The core snap should allow to disable this service through a services config option (similar to how we disable ssh or rsyslog today) that can be pre-seeded from a gadget snap configuration option.