1. The first a replica (or DO in general) is created in a region it's assigned a specific datacenter

The first a replica (or DO in general) is created in a region it's assigned a specific datacenter location in the region randomly. Then, every time that specific replica comes online it will start there, if that DC is available and healthy, or moved elsewhere in the same region temporarily. This behavior is what happens NOW, and is NOT a guarantee we provide. We want to be flexible to move replicas in the future, since that's the plan, to dynamically create and move replicas based on incoming traffic.
The main delay is fetching the DB from cold storage. That strictly depends on the size. It can be a few milliseconds or many seconds.
DB size is the main contributor.