HADR_THROTTLE_LOG_RATE_SEND_RECV_QUEUE_SIZE wait type
Since you are uploading to Azure SQL Database, this is likely to do with how fast they will let you push log records to the replicas you have. You can't configure those or decrease any throttling they're going to do; depending on service tier you're likely just going to have to resign yourself to a certain threshold on speed for bulk data transfer, whether you take this speed hit from their throttling, or changes you make yourself by injecting batching / delays to indirectly reduce log writes per time frame. 

In other words, the wait isn't documented, and there's nothing you're going to be able to do about it anyway, except maybe move up to a better service tier. But even that is no guarantee, since I am sure there is throttling across the board, and it might even always be the same regardless of tier.