Azure cross-region snapshot copy speed is throttled by source disk tier

Snapshots in Azure are stored in Azure Storage independent of the source disk, so naively you'd expect cross-region snapshot copy speed to be the same regardless of source disk tier. It is not. Empirically: a snapshot of a Premium SSD source disk (64GB OS) copied at ~70MB/s and hit 100% in ~15min; a snapshot of a Standard HDD source disk (128GB data) copied at <10MB/s and was still at 16-24% after an hour. Same target region, same subscription, same time, both with --copy-start true. The throttle appears tied to the original disk's performance tier even though the snapshot itself is decoupled storage. Mixed-disk VMs migrating cross-region will have OS-disk wall-clock dominated by Premium and data-disk wall-clock blown out by Standard. Workaround: either upgrade the source data disk to Standard SSD or Premium SSD before snapshotting, or for small-data scenarios skip the data snapshot entirely and rsync the data over the inter-VM IP path after the new VM is up (often faster for under-50GB working data than waiting for Standard HDD snapshot copy). The --bandwidth-copy-speed Enhanced flag exists but is gated behind Microsoft.Compute/EnhancedProvisionedBandwidthCopy feature registration which currently returns 'feature does not support registration' for most subscriptions — likely requires sales contact or enterprise agreement.