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When you say the flavour has a 400GB disk, how much disk space does the image really use in Glance ?

In summary, what happens on boot:

Nova checks to see if the image exists locally (in cache) if not, it will download it from Glance. Assuming Glance is not running under any web server like Apache, this will eat some CPU on the server running Glance and will take some time if the instance is big enough.

If the image is cached locally (after you first create an instance, depending on your image cache configuration), then a copy is created from cache. This should be much faster, but it depends on the size of the actual image. If you have an image that's 100GB (the actual image, not the filesystem inside the image), then copying 100GB of data would take a long time.

If you have a single LVM for the entire OS (compute node) and ephemeral storage, when you create an instance from a cached image, the image needs to be copied locally from one place to another, this will also slow down instance creation.

Check what processes are running and using CPU/I/O on the compute node while you create an instance and if the disk I/O is maxed out (most likely).