@Chris Parr Welcome to Microsoft Q&A Forum, Thank you for posting your query here!
This article lists some common problems related to Azure file shares and provides potential causes and workarounds. Troubleshoot Azure file shares performance issues
Based on your scenario, we need to investigate closely I would recommend you to contact support, so If you have a support plan, I request you file a support ticket, else please do let us know, we will try and help you get a one-time free technical support. In this case, could you send an email to AzCommunity[at]Microsoft[dot]com referencing this thread as well as your subscription ID. Please mention "ATTN subm" in the subject field. Thank you for your cooperation on this matter and look forward to your reply.
Please let us know if you have any further queries. I’m happy to assist you further.
Please do not forget to "Accept the answer” and “up-vote” wherever the information provided helps you, this can be beneficial to other community members.