You are definitely not constrained by the maximum performance of any one hard drive. Writing and reading data simultaneously across multiple hard drives can reduce the I/O bottleneck and increase overall disc performance. This practice is called RAID 0 in an on-premise environment. RAID stands for Redundant Array of Inexpensive Disks. But in the cloud, referred to as disc striping.

The concept is pretty simple, instead of a read or write operation carried out by one disc controller on one hard drive, the I/O operation is split amongst multiple hard drives so it can be carried out simultaneously. This means that a file will be split across multiple hard drives. This divide and conquer strategy is managed by the operating system and is invisible to other applications, as a pool of striped disks appears as one volume.

Because disk access is so slow relative to CPU and RAM, the benefits of multiple hard drives working simultaneously in unison far outweighs the cost of managing disk striping. Higher RAID levels that offer redundancy are unnecessary in the Azure environment as multiple synchronous disk replication protects against disk failure. To create a striped disk set or pool, use the Storage Spaces utility within Windows to add disks connected to your VM to the pool.

Azure's documentation talks about disk striping with Premium SSDs, but you can stripe with Standard SSDs and HDDs, so it may be possible to achieve better IO performance using cheaper media, but that would be something you would have to test for yourself. Disk striping is not available with Ultra disks.