EBS direct APIs verify slow

I use EBS direct APIs to access the contents of an EBS snapshot, when I called the .NET API AmazonEBSClient.GetSnapshotBlockAsync to get the data in block, the response was very slow, almost 5 minutes after I called the GetSnapshotBlockAsync, there was no exceptions and throttling errors during the call. The size of the snapshot is a little big, which is at TB level, I was wondering why the API is so slow? any method to speed up?

Thanks,