By and large copying the image currently dominates start up time for GCE.
Things you can do to minimize this:
- Use a pre-created persistent disk to boot from. In this case, there is no image propagation time.
- Use one of our stock images (that are pretty small) or, if you create a custom image, don't add more bits than you need.
Typically, the instance type and attached disks won't affect time dramatically.