Download speeds of uncached items when using Front Door in front of Blob Storage very slow even from Azure VMs in same region

Question

I have the following set up:

• Azure Blob Storage in South Africa North
• Azure Front Door with CDN back by the Blob Storage
• Azure managed Windows 11 Virtual Machine in South Africa North

When I download a 120MB (megabyte) file which happens to be uncached by the CDN, it downloads at less than 2MB/s (even from the Windows 11 Azure provisioned VM), when I retry the same download immediately afterwards (i.e. the cache is now hot) it downloads at 10s of MB/s.

The same behaviour happens on my home fibre connection which is also within 100km range of datacenters.

This <2MB/s performance on cold cache when I'm in the same geographical region seems terrible to me, what is the expected speed?

Answer 1

I've finally received a supposedly authoritative response from Microsoft on the matter I opened the ticket about.

The support agent has told me it is normal that downloading of cold/uncached items through Azure Front Door CDN backed by Azure Blob storage in South Africa North is at the slow speed of less than 2MB/s:

Them:

Hello Jonathan,

I hope you are doing well!

I sincerely apologize for the significant delay in our response, which was necessary to conduct further internal testing.

After a comprehensive review, we have determined that the behavior you are experiencing is typical for this type of operation.

This is primarily due to the connection not being entirely directly, as it must pass through Azure Front Door. This process also involves distributing the cache among point-of-presence (POP) servers, which inevitably impacts the > operation's speed. Let me provide you with documentation covering that matter:

How caching works in Azure Content Delivery Network | Microsoft Learn

Me:

So to be clear, Azure Front Door maxes out at less than 2MB/s (16Mbit/s) for uncached items even when everything is on Microsoft’s own servers?

Them:

Hello Jonathan,

Thank you for getting back to me.

These values may vary by region, but those particular ones apply for South Africa North.