Why is cold starts for managed functions in Azure Static Web App so much slower than "bring your own" functions?

Roger Sæle 1

We observe cold starts for managed functions in an Azure Static Web App to be very slow. When deploying the same function app to a dedicated Function App instead and linking it to the Static Web App, cold starts are much quicker. First requests when cold can take 15-30 sec for managed functions, whereas with "bring your own" it only takes around 1-3 sec. Why is this? Is it possible to reduce the cold start somehow?

You might say, just use "bring your own", but to leverage Distributed Functions (in our fully globally distributed web application) we need to stick with managed functions.

Ryan Hill 29,481 Reputation points Microsoft Employee

2025-01-08T19:28:15.8733333+00:00

Hey @Roger Sæle it's really tough to say what the issue could be. Have you checked any of the detectors on under the Diagnose and solve problems blade? Using App Insights may help too. It'll show what could be adding latency to the initial request.
Roger Sæle 1 Reputation point

2025-01-09T13:45:53.7833333+00:00
Diagnose and solve problems:

Under Availability and Performance I get:

Unable to fetch Static App Config. Please try again later

Under Configuration and Management I get a bunch of:

Your site has not been visited in a while. We are fetching your runtime configuration. Please come back in a few minutes.

Trying again later does not help. I do however also get the same with our other SWAs.

I have custom auth and the API contains a function to get roles. I can see from logs in App Insights that requests to the GetRoles function will take a long time during cold starts. However if I leave the session open and just refresh after the function app has gone to sleep, I am already authenticated and thus it is request to other function which will take a long time. Hence it is not the GetRoles function that is the issue. Also, like mentioned, if I deploy the exact same code to a dedicated function app, cold starts are around 10 times quicker.

Looking in traces I see execution time of the GetRoles function is only around half a sec at cold starts, but there is a delay from the time I access my web app until "Initializing warmup extension" happens (21 sec for instance). This is the route of the issue it seems. What does it do before "Initializing warmup extension"? And why is it so much slower with managed compared to "bring your own" functions?
Roger Sæle 1 Reputation point

2025-01-09T14:44:47.3533333+00:00

I also tried to strip down the function app to a bare minimum with only the GetRoles function, but it is still slow at cold starts.
Ryan Hill 29,481 Reputation points Microsoft Employee

2025-01-10T17:06:35.73+00:00

Thanks for the clarification @Roger Sæle, I'll look into "Initializing warmup extension" and see what I can dig up. In the meantime, can you respond to the private message so I can gather some additional details from you.

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Why is cold starts for managed functions in Azure Static Web App so much slower than "bring your own" functions?

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