How-to improve data loading performance on SQL Managed Instance with General Purpose service tier

I think something may be missing from this document in regards to disk performance and DB size on Azure SQL Managed Instances, we had a 400GB database that was running into disk write performance issues, so we expanded it to 1.1TB in expectations that it would scale according to what is explained above.  Our benchmark batch of write intensive queries took longer to execute the following day rather than less time to execute.  So simply expanding the size of the DB file did not seem to help with performance.